The Black-headed gull (Larus ridibundus), a gull species commonly seen on shores of Küçükçekmece lake located in Marmara Region, does not exhibit sexual dimorphism as it has monomorphic feather traits like the other gull species. In order to perform sex discrimination in gulls, the effective discriminant function analysis (DFA) method is used for the measurements based on the external morphology and determining the variables discriminating the two sex groups. The head morphological measurements and the body weights of 28 adult Black-headed gulls were obtained. One discriminant function was determined based on the results. The accuracy rate in two sexes was determined to be 82.1% (85.7% female, 78.6% male) using the formula we obtained as a result of the discriminant function analysis in which the head length, determined to be more significant compared to the other variables, was used. The aim of the study is to assess the sexual dimorphism in population of Black-headed gull in Küçükçekmece Lake and its surrounding located in Marmara Region and represent a reliable method to make a sex discrimination along with discriminant analysis.