In this paper, clarification of the fastest stroke in freestyle swimming by simulation was attempted. That is, the front crawl swimming was optimized for swimming speed and propulsive efficiency using Genetic Algorithm (GA). First, GA was integrated into our swimming human simulation model SWUM, which was developed for analyzing swimming motion. The design variables in the optimization were joint angles of the upper limbs. In addition, in order to obtain reasonable results and to make the computation more efficient, three constraints were imposed on the optimizing algorithm. Then, the fastest and most efficient strokes were numerically solved. Both the obtained strokes have their own features distinguished from the conventional strokes. From the analyses of the joint torque and fluid force, it was found that the fastest stroke is explained as 'full power during whole stroke', while the most efficient as 'full power in efficient moment'