The purpose of this study was to investigate the effects of age and sex on reaction speed. Experiment was designed to examine the changes of reaction time as a function of age(7,10,13,16,19), sex(male, female) and task complexity(1,2,4) from the developmental viewpoint. Subjects for the Experiment were 70 right-handed males and females and they were equally divided into five age groups: 7 years, 10years, 13years, 16years, 19years old groups. In all condition, task was simple and choice reaction and apparatus was Multipurpose Reaction Timer. Subjects were ordered to press the left button with right hand as rapidly as possible after a few seconds of foreperiods. Experiment was perfomed in a 5 x 2 3-factors within subject design. the conclusions were as follows: First, reaction time increased as the increase of task age. Second, reaction time increased as the increase of task complexity. Third, reaction time of man is geherally shorter than that of female.