Abstract: This paper reports histological and ultrastructural changes in the gonad during sex reversal from female to male in Monopterus albus. During the sex reversal the follicle cells became hypertrophic and were filled with secreta. The microvilli, zona radiata, and mitochondria of the oocytes degenerated. The yolk spheres were liquefied. After the yolk granules were digested and absorbed by hypertrophic follicle ceils, a round nodule with orange-coloured substance was finally formed. With the degeneration of the ovary, the spermatids and the testis were gradually formed.