Virtual Reality (VR) is a means of acquiring professional skills in various fields. Estimating cognitive load during VR training can help optimize learning materials. Although cognitive load is known to cause pupil dilation, few papers have reported the relationship between cognitive load and pupil diameter during VR multi-tasking. In this paper, pupil diameter during mental multi-tasking was measured in virtual environments (n = 16). Concurrent Go/NoGo tasks and mental arithmetic tasks were used for multi-tasking in VR. Go/NoGo stimuli were presented as images on the display, and participants responded by VR controller. Simultaneously, participants listened to arithmetic problems as computer voice and answered orally. Subjective cognitive load and performance index were also obtained. In addition to comparing pupil diameter at different task levels, correlation analysis was performed. The results indicate that pupil diameter dilates more when tasks were difficult by statistical tests. Correlation tests show that pupil diameter correlates directly with subjective cognitive load and correlates inversely with performance index. The consequences support previous studies and suggest the feasibility of evaluating cognitive load during VR training using pupil diameter.