The study of fish reproductive biology and fecundity is essential for effective management and conservation of fishery resources in an aquatic environment. Clarias gariepinus is a key fish species in Nigeria which contributes greatly to the nation’s aquaculture sector as a result of its economical and nutritional values. Experimental trials were conducted to evaluate the fecundity and egg size of Clarias gariepinus brooders (60 - 120g) in Ero reservoir. Fifteen mature female fish were caught using gill nets and long-line gears. In addition, fishermen's catch was also sampled to increase sample size and fish size. Fish were dissected and their ova sac removed, weighed and preserved in 5% formalin to prevent quick egg-freezing action before counting. An ocular micrometer (Mg=0.019 mm) was used to obtain the egg size. A unit increase in the length of the fish led to 0.52g increase in the body weight. Between total length and fecundity, there is a significant connection with p-value less than the level of significance at (p < 0.05). Fecundity were found to be related to the body weight of the fish. The result indicates that the relative fecundity increased with increasing body weight and total length suggesting that the body weight of fish plays an essential role in fecundity of fish. Also, Clarias gariepinus has a significantly high fecundity in Ero Reservoir thus there is need for monitoring of the fish resource in Ero Reservoir to avoid over fishing of the species and enhance sustainable fishery management of the reservoir.