The Caspian Sea is the largest lake in the world and is facing an increasing risk of contamination. Sea-based and land-based hydrocarbons inputs are the most critical pollutants of the Caspian Sea. In the present study, n-alkanes and related diagnostic ratios were used to identify the sources of hydrocarbon contaminants in surface sediments along the southern coastal area of the Caspian Sea in Mazandaran province. 27 surface sediment samples were collected from a depth of 5 meters under sea level. Contaminant’s compositions were extracted by Soxhlet apparatus and analyzed by Gas Chromatography-Mass Spectrometry. According to the results, the total concentration of n- alkanes (ΣHC) ranged from 5.2 μg g-1to 690.7 μgg-1. The S20 (selected station in Noshahr port) had the highest concentration, and most of the stations have levels in a moderate range. Five diagnostic ratios are used to distinguish petrogenic and biogenic contaminants in shorelines of the Mazandaran province.ETC...