In this study, size compositions of Aurelia aurita (Linnaeus, 1758), Pleurobrachia pileus (O. F. Müller, 1776) and Mnemiopsis leidyi A. Agassiz, 1865 were presented seasonally between March 2015 and February 2016 at the Sinop Peninsula of Black Sea. Sampling was carried out monthly at the 6 stations by plankton net (50 cm diameter mouth opening, 500 µm mesh size for horizontal tows, 210 µm mesh size for vertical tows). Small sized individuals of A. aurita, M. leidyi and P. pileus were found dominant in summer. Bigger size individuals were observed for A. aurita in spring, M. leidyi in winter, and P. pileus in autumn. A. aurita ˂4 cm was determined as 57%, M. leidyi ˂2 cm was 58% and P. pileus ˂ 0.6 was determined as 67%. Maximum and minimum lengths were measured 28 cm and 0.5 cm for A. aurita, 2 cm and 0.2 cm for P. pileus, 9 cm and 0.2 cm for M. leidyi, respectively. The size distribution of A. aurita, M. leidyi and P. pileus showed significantly different (ANOVA, p˂0.05) between seasons.