This study was conducted to assess the effect of pupil size on dynamic visual acuity (DVA). 60 young healthy men (M = 28.1 yr., SD = 3.9) with normal vision were divided into three age-matched groups by pupil size: dilated (n=20), unchanged (n = 20), and constricted (n = 20). DVA was measured binocularly with free-head viewing before and at 30 min. after each drop was instilled. Each of the three groups got a different amount. The sizes of the constricted, unchanged, and dilated pupils were 2.8 mm (SD=0.5), 4.1 mm (SD=0.3), and 7.8 mm (SD = 0.5), respectively. The pupil size × DVA interaction was significant (F2,114 = 6.07). DVA in the constricted pupil decreased, but that in the dilated pupil increased (paired t test). DVA in the unchanged pupil did not change significantly (paired t test). Pupil size is possibly one of the factors which may affect DVA measurement.