A mandoline slicer, comprising: a frame having two opposed side walls and a food support surface positioned between the two opposed side walls; a blade opening provided on the food support surface; a slicing thickness adjustment mechanism, comprising a blade assembly axle which passes through two corresponding blade assembly axle openings at the two opposed side walls of the frame so that each of a first end and a second end of the blade assembly axle protrudes outward from the corresponding side wall of the frame and connects to a knob rotatably mounted at the corresponding side wall; an eccentric cam is provided near each of the first and the second end of the blade assembly axle; a blade assembly movably received within the frame; the blade assembly is spring-loaded to be biased away from the food support surface; the blade assembly comprises two opposed blade assembly side walls and a blade seat positioned between the two opposed blade assembly side walls, and each of the two opposed blade assembly side walls has a corresponding notch at a lower side thereof; the eccentric cams at the first end and the second end of the blade assembly axle are configured to abut against the notches so as to overcome the spring biasing force exerted to the blade assembly and move the blade assembly; characterized in that: the blade assembly is mounted with a cutting blade having two longitudinal cutting edges for bi-directional food slicing; the blade seat of the blade assembly positioned between the two opposed blade assembly side walls receives the cutting blade; the blade assembly moves vertically relative to the food support surface between an operating position where the cutting blade is exposed above the food support surface for a vertical distance for slicing and a resting position where the cutting blade is positioned coplanar with or below the food support surface. a frame having two opposed side walls and a food support surface positioned between the two opposed side walls; a blade opening provided on the food support surface; a slicing thickness adjustment mechanism, comprising a blade assembly axle which passes through two corresponding blade assembly axle openings at the two opposed side walls of the frame so that each of a first end and a second end of the blade assembly axle protrudes outward from the corresponding side wall of the frame and connects to a knob rotatably mounted at the corresponding side wall; an eccentric cam is provided near each of the first and the second end of the blade assembly axle; a blade assembly movably received within the frame; the blade assembly is spring-loaded to be biased away from the food support surface; the blade assembly comprises two opposed blade assembly side walls and a blade seat positioned between the two opposed blade assembly side walls, and each of the two opposed blade assembly side walls has a corresponding notch at a lower side thereof; the eccentric cams at the first end and the second end of the blade assembly axle are configured to abut against the notches so as to overcome the spring biasing force exerted to the blade assembly and move the blade assembly; characterized in that: the blade assembly is mounted with a cutting blade having two longitudinal cutting edges for bi-directional food slicing; the blade seat of the blade assembly positioned between the two opposed blade assembly side walls receives the cutting blade; the blade assembly moves vertically relative to the food support surface between an operating position where the cutting blade is exposed above the food support surface for a vertical distance for slicing and a resting position where the cutting blade is positioned coplanar with or below the food support surface.