The objective of this study was to investigate the effect of pen and group size on behavioral characteristics of Hanwoo steers. Forty-eight beef steers, all 15 months of age, were used in this study, and were assigned to 3 groups: control (4/35.28 ㎡), treatment 1 (8/70.56 ㎡), and treatment 2 (12/105.84 ㎡). In addition, behavioral characteristics were analyzed every 2 days in each month using CCTV cameras. Each behavior of the treatment group was observed significantly more frequently (p<0.05) than the corresponding behavior of the control group in summer, especially feeding behavior. In addition, no significant difference was observed between the treatment groups. Locomotion was observed for a significantly longer duration in the control group than in the treatment groups (p<0.05), and resting was observed for slightly longer in the treatment groups than in the control group. Fighting behavior, related to social status, was observed significantly more frequently in the control group than in the treatment groups (p<0.05). In winter, feeding behavior was observed for a significantly longer duration in the control group than in the treatment groups (p<0.05) On comparing the behavior of Hanwoo steers during different seasons, we found that the steers showed more resting behavior during summer and more feeding behavior and movement during winter. The above results suggest that the behavioral characteristics of Hanwoo steers depend on the pen and group size, based on the higher frequency of fighting, rubbing, and scratching observed in the treatment groups than in the control group. The results also suggest that the behavioral characteristics of Hanwoo steers depend on the environmental conditions, as shown by the seasonal differences in behavior. Under conditions of high temperature stress, Hanwoo steers showed increased lying down, drinking and grooming behavior to counter this stress. The findings of this study can be applied to management practices for animal welfare and producticon of cattle in both growing and fattening periods.