Abstract: Sthenoteuthis oualaniensis is an economically important Cephalopod in the Indian Ocean, which has a large amount of resources and rapid biological turnover. Fecundity provides important data for reflecting the survival ability of species, population reproduction, population biomass and other indicators. Research on fecundity can provide insight into the adaptive characteristics of organisms or populations to survive in the environment. In order to deeply understand the effective fecundity characteristics of male S. oualaniensis in the Northwest Indian Ocean, according to the samples of S. oualaniensis in the Northwest Indian Ocean from November to December 2021 and from March to May 2022. The data of individual mantle length, body weight, sexual maturity, gonadal weight, number of spermatophores, length of spermatophores and length of spermatophore sacs were measured, the effective fecundity characteristics, the changes of spermatophoric sac and spermatophore with gonad development were analyzed. The results showed that the mantle length of male S. oualaniensis ranged from 127 to 275 mm, and the body weight ranged from 87.1-606.1 g. The gonad index was 0.20%-2.73%, and the spermatophoric complex index was 0.08%-2.63%. The individual fecundity ranged from 1-220, and the relative fecundity of mantle length ranged from 0.006-1.073 mm each. The length of spermatophoric sac ranged from 49.32-171.60 mm, and the weight ranged from 0.082-5.141 g. The length and weight of spermatophoric sac increased with the increment of individual mantle length and body weight. The length of spermatophore was 5.45-70.99 mm, and the weight of spermatophore was 0.2-19.5 mg, they showed a power function relationship. The length and weight of spermatophore scaled up with growing carcass length and body weight. The above results showed that the gonad index and spermatophoric complex index of male S. oualaniensis in the Northwest Indian Ocean magnified with gonad development. Individual effective fecundity, spermatophoric sac and spermatophore increased with the development of gonad, and had significant functional relationship with individual carcass length and body weight. This study provides fundamental data on the reproductive biology of male S. oualaniensis, which is crucial for the assessment and sustainable management of this resource in the Northwest Indian Ocean.