A folding balance bike comprising: a frame including a body portion, a groove extending through the body portion, and a saddle connected with the body portion; a front wheel assembly pivotally mounted on one end of the frame, with the front wheel assembly including a front wheel; and a rear wheel assembly pivotally mounted on the other end of the frame opposite to the front wheel assembly, with the rear wheel assembly including a rear wheel rotatably mounted therein; wherein the rear wheel assembly is pivotable with respect to and able to be received into the groove of the frame to adjust a wheelbase of the folding balance bike to cause the folding balance bike operable between an operative state and a stored state, with the wheelbase defined as a horizontal distance that extending between centers of front and rear wheels (21, 31); and wherein when the folding balance bike is in the operative state, the rear wheel of the rear wheel assembly is exposed from and securely disposed at the other end of the frame; and wherein when the folding balance bike is in the stored state, the rear wheel of the rear wheel assembly is received in the groove of the frame; and wherein the wheelbase of the folding balance bike arranged in the stored state is shorter than that arranged in the operative state to facilitate transport and storage; and wherein the body portion includes a locking hole extending therethrough, with the rear wheel assembly including the rear wheel rotatably mounted on a distal end of a swing fork thereof, with the swing fork including a connecting portion and two arm portions respectively extended from two sides of the connecting portion, with a plurality of locking apertures extending through the connecting portion, with one of the plurality of locking apertures able to be aligned and interconnected with the locking hole of the body portion, with a first fastening assembly engageable into one of the plurality of locking apertures of the swing fork and the locking hole of the body portion to cause the rear wheel assembly selectively fixed with respect to the frame; and wherein a slot is formed at one distal end of the body portion opposite to the saddle, with the connecting portion of the swing fork engaged into the slot of the body portion; and wherein the body portion includes a pivoting hole extending therethrough and interconnecting with the slot thereof, with the swing fork including first and second pivoting apertures (324,325) respectively extending through two opposite ends of each of the two arm portions thereof, with two second fastening assemblies respectively engaged into the first pivoting aperture of the swing fork and the pivoting hole of the body portion and engaged into the second pivoting aperture of the swing fork. a frame including a body portion, a groove extending through the body portion, and a saddle connected with the body portion; a front wheel assembly pivotally mounted on one end of the frame, with the front wheel assembly including a front wheel; and a rear wheel assembly pivotally mounted on the other end of the frame opposite to the front wheel assembly, with the rear wheel assembly including a rear wheel rotatably mounted therein; wherein the rear wheel assembly is pivotable with respect to and able to be received into the groove of the frame to adjust a wheelbase of the folding balance bike to cause the folding balance bike operable between an operative state and a stored state, with the wheelbase defined as a horizontal distance that extending between centers of front and rear wheels (21, 31); and wherein when the folding balance bike is in the operative state, the rear wheel of the rear wheel assembly is exposed from and securely disposed at the other end of the frame; and wherein when the folding balance bike is in the stored state, the rear wheel of the rear wheel assembly is received in the groove of the frame; and wherein the wheelbase of the folding balance bike arranged in the stored state is shorter than that arranged in the operative state to facilitate transport and storage; and wherein the body portion includes a locking hole extending therethrough, with the rear wheel assembly including the rear wheel rotatably mounted on a distal end of a swing fork thereof, with the swing fork including a connecting portion and two arm portions respectively extended from two sides of the connecting portion, with a plurality of locking apertures extending through the connecting portion, with one of the plurality of locking apertures able to be aligned and interconnected with the locking hole of the body portion, with a first fastening assembly engageable into one of the plurality of locking apertures of the swing fork and the locking hole of the body portion to cause the rear wheel assembly selectively fixed with respect to the frame; and wherein a slot is formed at one distal end of the body portion opposite to the saddle, with the connecting portion of the swing fork engaged into the slot of the body portion; and wherein the body portion includes a pivoting hole extending therethrough and interconnecting with the slot thereof, with the swing fork including first and second pivoting apertures (324,325) respectively extending through two opposite ends of each of the two arm portions thereof, with two second fastening assemblies respectively engaged into the first pivoting aperture of the swing fork and the pivoting hole of the body portion and engaged into the second pivoting aperture of the swing fork.