Abstract: The group sizes were 4, 8 and 12 pigs/pen, providing a floor allowance of 1.50, 0.75 and 0.51 m²/pig, respectively. The pigs that were reared in a group size of 12 pigs/pen ate, drank, walked, mouth piped, rooted the floor, rooted the pipe and pawed more significantly often (p<0.05) than did the pigs that were reared in group sizes of 4 and 8 pigs/pen. These feeding behaviors and the high temperature of Thailand caused high rectal and skin temperatures which were the highest in the group size of 12 pigs/pen. Moreover, the high temperature and the increased group size may cause heat stress which would decrease the Average Daily Gain (ADG). The largest group size of 12 pigs/pen presented the lowest ADG in both growing and finishing phases which significantly differed (p<0.05) compared to the group sizes of 4 and 8 pigs/pen. In addition, the high temperature and the feeding frequency in the group sizes of 8 and 12 pigs/pen provided a better back fat thickness that was significantly less than that of the group size of 4 pigs/pen (p<0.05).