A motorcycle frame, comprising: a main tube having a main body having two opposite sides;; and an outer surface; a head tube; and a connecting tube extending laterally and having two opposite ends, wherein the head tube and the connecting tube are respectively and securely mounted on the ends of the main body; two curved supporting tubes located below the main body, each supporting tube having an upper segment having a distal end and an axis, wherein the distal ends of the two upper segments are securely mounted on the outer surface respectively at the two opposite sides of the main body; and a lower segment connected to the upper segment and curving upward relative to the axis of the upper segment, wherein an engine mounting space is formed below the two lower segments; two auxiliary tubes respectively located at the two opposite sides of the main body, respectively and securely connected to the two opposite ends of the connecting tube, each auxiliary tube having a first end, wherein the first ends of the two auxiliary tubes are respectively and securely mounted on the two supporting tubes; and a first rigid structure formed at where the main tube, the supporting tubes, the connecting tube, and the auxiliary tubes surround, and having a receiving space formed through the first rigid structure wherein the main body comprises an inclined segment, and the motorcycle frame being characterized in that the main body has a horizontal segment having a distal end on which the head tube is securely mounted, wherein the upper segments are mounted on the horizontal segment; and an inclined segment securely connected to the horizontal segment, curving downward relative to the horizontal segment and has a distal end on which the connecting tube is securely mounted. a main tube having a main body having two opposite sides;; and an outer surface; a head tube; and a connecting tube extending laterally and having two opposite ends, wherein the head tube and the connecting tube are respectively and securely mounted on the ends of the main body; two curved supporting tubes located below the main body, each supporting tube having an upper segment having a distal end and an axis, wherein the distal ends of the two upper segments are securely mounted on the outer surface respectively at the two opposite sides of the main body; and a lower segment connected to the upper segment and curving upward relative to the axis of the upper segment, wherein an engine mounting space is formed below the two lower segments; two auxiliary tubes respectively located at the two opposite sides of the main body, respectively and securely connected to the two opposite ends of the connecting tube, each auxiliary tube having a first end, wherein the first ends of the two auxiliary tubes are respectively and securely mounted on the two supporting tubes; and a first rigid structure formed at where the main tube, the supporting tubes, the connecting tube, and the auxiliary tubes surround, and having a receiving space formed through the first rigid structure wherein the main body comprises an inclined segment, and the motorcycle frame being characterized in that the main body has a horizontal segment having a distal end on which the head tube is securely mounted, wherein the upper segments are mounted on the horizontal segment; and an inclined segment securely connected to the horizontal segment, curving downward relative to the horizontal segment and has a distal end on which the connecting tube is securely mounted.