This study is aimed to evaluate how two different lengths of work, i.e. 30 minutes and 90 minutes, are inflence the psycho-physiological reaction of eight young subjects and their working behaviour before and during work. It is found that the time length factor in this experiment had no relationship with their anxiety before work. It can also be said that the deterioration of work performance and the activating level of parasympathetic nerve system in the case of 90 minutes work are larger than in the case of 30 minutes work. As for the subjects' controllability of arousal level of cerebral cortex, it can be said that the inferior performance group finds it more difficult to control than superior performance group does, which is likely to induce the excitement of the parasympathetic nerves of the subjects. In conclusion, emotional factor of subjects feeling a certain work time shorter or longer should be taken into consideration as an important index when work length of work time is evaluated from a view point of work load and fatigue.