To determine the fecundity and population composition of Gammarus (Pontogammarus maeoticus (Gurianova,1951) ) in coast line of the Caspian Sea, monthly sampling was conducted in ten stationg (between Babolsar to Fereydunkenar) from August 2013 to April 2014.A total of 1906 P.maeoticus speciments, male populationcomposition (673), female (873) and immature (360), respectively. Kruskal-Wallis test based on the average number of Gammarus in stations (χ2 = 14.8; p<0.011) and months (χ2 = 17.6; p<0.040) showed significant different. The average length of immature, male and female was 4.1, 8.5 and 8.3 mm and weight 0.0068, 0.0303 and 0.0282, respectively. Males have more weight and length than the female and the mean length and weight of the two sexes was statistically significant (length: t-test=2.05; p<0.041 Weight: t-test=2.81; p<0.005, respectively). Gammarus Length and weight equation was very strong based on exponential relationship (p<0.001; R2= 0.80) and slope of line b was 2.1205. The minimum average fecundity was 59/0±0/6 and a maximum of 71/0±4/16. Kruskal-Wallis test between the mean fecundity in different months showed a significant difference (p<0.001).The results show that the overall ratio of male: female was 77/0: 1 which showed completely dominant of females (p<0.001).Also compared the frequency of male and female at different months showed that only in August and March females are dominant statistically (p<0.05).