In this study fatty acid contents, because of the importance in human’s life, and their seasonal variations of a dominant bivalve of Haleh Creek were determined for the first time. Fatty acid identification was done by GC-MS (Gas Chromatography–Mass Spectrometry) method. After collecting and dissecting the species from the shells, samples weighed and frozen for further experiments. All the samples esterified and made ready for injection to the GC-MS. Temperature as an important environmental parameter monitored monthly, variability of the fatty acid components studied in two seasons, and the effect of temperature changes on seasonal variations of the fatty acids were studied by statistical analysis. In conclusion sixteen fatty acids identified in Callista umbonella, including twelve saturated and four unsaturated, which the most important were oleic, palmitic, myristic, hexadecanoic and nonadecanoic acids while saturated fatty acids were dominated over unsaturated ones. The most abundant saturated fatty acid was palmitic acid and unsaturated one was oleic acid in this species. Although fatty acids’ seasonal changes did not show any significant difference and also any significant correlation with temperature in the species in this area, but in general fatty acid amounts might be vary in different temperatures and it could be related to many environmental and biological factors in species.