Abstract: A total of 100 one-day-old healthy AA broilers and 120 one-day-old healthy Nongchang Ⅱ layers were selected and reared under the same feeding and management conditions. Ten AA broilers were weighed and anatomized at the age of 1, 3, 5, 7,14, 21, 28, 35, 42, 49 days, and the thymus samples were collected and weighed. The thymus index was calculated and paraffin sections of thymus tissues were prepared. The histological structure of thymus tissues were observed under light microscope. Ten Nongchang Ⅱ layers were also weighed and anatomized at the age of 1, 5, 7, 14, 21, 28, 35, 42, 56, 70, 84, 98 days, and the subsequent treatment was the same with AA broilers. The results showed that for AA broilers, the higher thymus index was observed at the age of 5（4.62） days and 42（5.09） days, and the highest thickness of thymic cortex and the biggest size of thymic corpuscle was observed at the age of 42 days and 5 days respectively. For Nongchang Ⅱ layers, higher thymus index and the highest thickness of thymic cortex was observed at the age of 56 days. Combined the analysis results of thymus index, thymic corpuscle thickness and thymic cortex size, the AA broilers has higher immunity level at the age of 5 days and 42 days, the Nongchang Ⅱ layers has higher immunity level at the age of 56 days. The presence of immunity peak period of the layers was obviously later than the broilers, which was associated with the fact that the growth cycle of layers was longer than broilers.