Abstract: The microstructure and ultrastructure of thyroid gland of 2-month-old African ostrich chicks were observed by light microscope and transmission electron microscope technique. The results showed that the thyroid gland of ostrich chicks consisted of follicles and parafollicular cells. The follicles were different in size, and were mainly composed of simple cuboidal epithelial cells. The diameter of follicles ranged from 20.0 to 280.0 μm, and with an average of 69.9 μm. The height of follicular epithelial cells were from 5.0 to 15.0 μm, and were 9.4 μm on average. Under transmission electron microscope, the follicular epithelial cells could be functionally classified into type A and B. The shape of type A epithelial cell was cuboidal or low columnar, but that of type B epithelial cell was squamous or low cuboidal. The nucleus of type A was bigger and round or oviform, and the euchromatin in them was abundant, but the nucleus of type B was smaller and elliptic, and there was much heterochromatin in it. In cytoplasm of type A, there were a large number of RER, mitochondrion, Golgi complex and lysosome; but in the cytoplasm of type B, those organelles were underdeveloped. The parafollicular cells in the thyroid gland of ostrich chicks were oviform or polygonal in shape, lesser in number and bigger in volume. The parafollicular cells located among the follicular epithelial cells or between the follicles, and there were a number of RER, mitochondrion, Golgi complex and secretory granule in the cytoplasm. The above results suggested that the function of thyroid gland in 2-month-old ostrich chicks is active comparatively, could synthesize and secrete a large quantity of thyroid hormone and some certain quantity calcitonin.