In. order to evaluate the effects of number and method of visual stimuli on simple reaction time and determine the proper number of trials, simple reaction time of the computerized test of NTOS (Neurobehavioral Test for Occupational Screening) was carried out on 240 medical students. The prominent difference was found between male and female on simple reaction time. Regarding the effect of number based on the parameters of measures and the purpose of test itself, 64 trials was considered as proper number. The mean reaction time was different according to the methods of visual stimuli, such as color and size(p<0.05). The larger and more intence visual stimuli were, the faster sim-ple reaction time was. In respect to sex, that difference was similar. But, there was no difference depending upon whether the each result of each trial was showed on the monitor. The subjects, however, showed much interest when the result showed. Further studies, including the various variables on visual stimuli, are required to compare among various studies.