Critical flicker frequency of 40 undergraduate students, 20 men and 20 women whose mean age was 19.3 yr., was measured under monocular and binocular viewing conditions, using the Lafayette Visual Perception Control with Display Unit. Half of the subjects received monocular treatment first and binocular treatment second, and the other half received the treatments in the reversed order. A 2-min. rest was allowed between these treatments. Analysis shows that mean CFF under the monocular condition was significantly lower than that under binocular conditions. The findings were discussed in terms of visual constraint and eye strain and fatigue under the monocular viewing condition.