Abstract: The bursa of Fabricius (BF) is a special organ in birds, being the site of mature B lymphocytes. This study investigated proliferating cell nuclear antigen (PCNA) expression in the BF in chicken early embryo development (5 to 18 days). The results showed that the BF of chicken embryos comes into being at the deepest point of the chicken cloaca at embryos of 6.5 days (E6.5), commencing from small vacuoles that develop into one large capsule at 8 days (E8), which continues growing from 8 to 10 days. The BF begins to exhibit small ruga in the capsule at E11, the ruga then become increasingly evident from 11 to 18 days, come through the whole BF, and the BF forms. When the BF profile formed at E9, with the strongest PCNA expression in the mucous epithelium, there is also broad PCNA expression in the lamina propria as the early embryo develops (9 to 18 days), the relative percentages of PCNAimmunopositive cell accounting for the total mucous epithelium were 100%, 100%, 100%, 99%, 99% and 99% at E9, E10, E11, E12, E14 and E18, respectively; the relative percentages of PCNAimmunopositive cell accounting for the total lamina propria cells were 97%, 93%, 71%, 55%, 56% and 20% at E9, E10, E11, E12, E14 and E18, respectively, which suggesting that lamina propria cells proliferation of BF decrease gradually in chicken early embryo development.