The study focuses on determining and identifying the fish crustacean parasitic infestation of Hasanlou reservoir Dam Lake in Naghadeh in 2011-12. Five species of fish including Cyprinus carpio Linnaeus, tecopharyngodon idella valenciennes, Leuciscus cephalus L., Capoeta capoeta Goldenstad, and Hypophthalmichthys nobilis were selected for the study. During 4 times sampling, 150 samples were taken and transported live to the laboratory on which different parts from skin, fins and gills were examined for their appearance and fish crustacean parasitic infestation by using a microscope and a loop. In order to preserve and accurately identify the samples, preservatives were used by maintaining the contemporary techniques and mechanisms. Regarding the type of analysis, which were done, it was found that 36% of the fish under study were somehow infected with crustaceans parasitic infection including Ergasilus, Lernaea cyprinacea, and Tracheliastes polycolpus. The degree of infestation was at its highest degree in summer with 22% and at its lowest level 9.33% in winter among which the highest and lowest frequencies of the infestation were related to the Ergasilus and Tracheliastes polycolpus with 42.59 and 20.37%, respectively.