Abstract: Abstract: Free expression of pig behavior is considered to be an important index of animal welfare. The free beds developed by China Agricultural University, where the sows are freely access to, can act as partitions to reduce the aggression behavior due to feeding competing on one hand, and on other hand provide a better lying environment for the sows. Effects of the free beds on improving the thermal comfort of the sows were confirmed, while the design on the bed width is not specified yet. The objective of this study was to evaluate the impact of unit width of swine bed and social hierarchy on behavior of pregnant sows to provide fundamental information of the bed design. In total, eighteen sows at about four weeks after breeding were randomly allocated into three special stimulation pens, and each pen was assigned with six pregnant sows. Every pen was equipped with six units of water-cooled cover, and the unit widths were 600 mm, 700 mm and 800 mm, respectively. Based on the results of wins and defeats of each sow during the first 48 h after mixing, they were individually labeled as dominant, subordinate, or submissive. The sows ranked 1st to 2nd places were defined as dominant, 3rd to 4th as subordinate and 5th to 6th as submissive. The results showed that lying and feeding behavior of group-housed sows was greatly affected by unit width of the bed and their social hierarchy. Generally, the sows spent more time lying inside the beds as the width increasing, and total lying time of each sow spent in the 600 mm width unit was significantly less compared with the 800 mm unit (P<0.05). The average duration of each lying behavior for subordinate sows in the swine bed were remarkably less than that of submissive sows (P<0.05). No significant difference was found on the duration of lateral lying that the dominant and subordinate sows spent in the 700 mm unit (P>0.05). However, it was significant less than that of submissive sows (P<0.05). Frequency of aggression behavior happened in the 600 mm width unit were significantly less than those of the 700 or 800 mm width unit (P<0.05). The frequency of replacement behavior during feeding increased progressively with increasing the unit width, and significant difference among different width units was observed (P<0.01). Compared with submissive sows, the frequency of attack and replacement behavior initiated by dominant and subordinate sows were significant higher (P<0.05). The occupying percentage of the first feeding unit by dominant sows was 62.5%, 50.0% and 100.0% in the three pens, respectively. The findings suggest that 800 mm unit widths of the bed is helpful for the sows to perform lying behavior but go against the feeding behavior, and the favorable resources of lying and feeding were typically occupied by dominant sows. Based on the finding of this study, it is suggested to appropriately increase the free beds width for better lying behavior of the sows, and meanwhile to design a locking component attaching on the ending part of the bed to reduce the aggressive behavior between the sows during feeding in the future work.