The main purpose of this study was to investigate gillnet catch composition and biodiversity caught by 130, 140, and 150 mm (STR) mesh sizes during the fishing season. This research was conducted in the fishing ground of Bushehr County located in the Persian Gulf from autumn 2018 to spring 2019. Overall, 23 families including 36 genera and 43 species were identified. Spring season with 42 species and winter season with 32 species showed the highest and lowest species biodiversity, respectively. Carangidae family with 7 genera and 8 species, Scombridae family with 3 genera and 4 species, and Dasyatidae with 3 genera and 4 species were the families with the highest species biodiversity. Scomberomorus commerson and Thunnus tonggol with 23.45% and 21.84% in autumn, 20.15% and 25.85% in winter, 21.9% and 19.78% in spring showed the most abundance percentages, respectively. The 130 mm mesh size and spring season had the highest abundance, diversity, and the number of caught species in comparison with other mesh sizes and seasons. Two-ways ANOVA results showed that the effects of mesh sizes and seasons on the calculated metrics were significant (P0.05). The highest and lowest Jaccard (J) and Sørensen (S) seasonal similarities were between autumn and spring (J: 0.805, S: 0.892) and autumn and winter (J: 0.561, S: 0.719), respectively. The results of non-metric multidimensional scaling analysis showed that 130 and 140 mm mesh sizes had overlap in terms of the fish abundance and catch composition. Cluster analysis also showed the highest similarity between S. commerson and T. tonggol, and Euthynnus affinis and Scomberoides commersonnianus.