The FEM was used to analyze the stress distribution of a motorcycle frame on the condition of normal running, starting up, turning and braking, respectively. Having been carrying on the 2000km of running test, the initial design motorcycle frame was found several cracks or fractures in the bending points of the square tube and in the welding seam, whose reasons were analyzed then. The metallographical structures of materials and crack parts of the frame were observed and analyzed and Q235. Based on the results mentioned above, the Q345 steel(Chinese steel mark) was used to replace the Q195 steel in the bending square tube and the prop of the steering column. And the welding and manufacture processes were optimized and improved. Finally, the motorcycle frame whose materials had been optimized could keep running in good condition and satisfy the demands after 16000km of running test.