Twenty-one lambs (35-day-old, 6.71 ± 0.15 kg) were used in a completely randomized design to study the effect of pen size on behavior and performance. Lambs were randomly allocated to 3 different pen sizes (small: 0.34 m2/lamb, medium: 0.48 m2/lamb, and large: 0.63 m2/lamb) located in a semiopen barn for 42 days after 35-day milk feeding. Daily dry matter intake and average daily gain were recorded. Maintenance behaviors were recorded based on the methods of scan sampling. The lambs kept in large pens showed an increase in dry matter intake compared to other groups (P < 0.05). However, there were no differences in average daily gain and feed conversion ratio among 3 groups. When pen size increased, lambs spent less time in the standing position and more time eating, ruminating, walking, playing, and resting (P ≤ 0.01). There were no significant differences between groups in term of drinking and other behavior parameters (defecation, urination, etc.). It was concluded that 0.34 m2/lamb may be an inadequate space allowance for weaned growing lambs, physiologically, but larger space provided for more varied and active behaviors.