Abstract: This experiment was conducted to study the dynamics and hormonal regulation of yolk sac utilization in different breeds of newly-hatched broilers. A total of 552 newly-hatched broilers (276 Beijing-You (BY) male broilers and 276 Arbor Acres (AA) male broilers) were used, and the birds of each breed were allotted randomly into six replicates of 46 birds according to body weight. After being weighed on days 0, 1, 3, 5, 7, 9 and 11 posthatching, 36 broilers of each breed were selected and killed. The results showed that: (1) Initial body weight and growth rate of BY broilers were significantly lower than those of AA broilers. (2) Yolk sac weight, air-dried material and total energy contents of yolk sacs in both BY and AA broilers decreased in a exponential fashion with age, but the coefficients of the exponential functions of BY broilers were smaller than those of AA broilers. (3) Leptin and insulin levels in yolk sacs varied with age and breed. During 3 days posthatching, leptin and insulin levels in yolk sacs of both BY and AA broilers increased with age. Compared with those of AA broilers, leptin and insulin levels in yolk sacs of BY broilers were higher (P ≤ 0.041 3) on days 0 and 3. The results suggested that the dynamics of yolk sac utilization were similar between BY and AA broilers with a decrease in an exponential way with age, and yolk sacs of BY broilers were utilized faster, which was in part due to the higher levels of leptin and insulin in yolk sacs of BY broilers compared with AA broilers.