The focus of this study is to investigate how personal display systems - a VDU (Visual Display Unit) and an HMD (Head Mounted Display) physiologically affect the body in virtual environments, and to evaluate differential effects of using the VDU and the HMD on physiological responses to mental stressful tasks (virtual reality flight simulation). As physiological variables, autonomic measures (heart rate, blood pressure), immune cells (leukocyte, neutrophil, lymphocyte), and hormones (catecholamine) were measured before and after experiments. Physiological data were measured in order to compare a level of mental stress on the VDU and the HMD. Increments in blood pressure (systolic (p<0.05), diastolic (p<0.1)), norepinephrin (catecholamine) (p<0.005), and neutrophils (p<0.2) of the group using the HMD showed a significant difference with the group using the VDU. Although, the heart rate was not statistically significant between two environments, differences of them quietly increased on the HMD more than on the VDU.