The aim of this investigation is the evaluation of heparin-induced thrombocytopenia (HIT) abundance and relevant factors risks in the patients receiving unfractionated heparin and low molecular weight heparin. This cohort study was conducted on two groups of patients who were hospitalized in general and vascular surgery department: receiving fractioned heparin group and receiving low molecular weight heparin group (LMWH). Platelet count was measured before treatment and on days 4, 7, 14, and 30 post-treatment. If in each of these steps there HIT criteria including platelet count less than 100000 or loss of 50% of base numbers, SRA diagnostic test would conduct and if positive, definite HIT was diagnosed. In this research 486 patients participated (234 patients in each group). According to heparin induced thrombocytopenia criteria, in this study only 3 patients had such criteria. Using SRA diagnostic test, HIT was determined in two out of three patients. The three patients were in the group of receiving UFH, also all of them were male and they receive heparin prophylaxis. There was no significant difference between 2 UFH and LMWH receiving groups. Heparin-induced thrombocytopenia (HIT) is not a common side effect of consuming low molecular weight heparin, but a deadly side effect and it is a strong risk factor in thrombotic events such as VTE; so prevention, diagnosis and treatment is very vital. Furthermore, the main key to reduce the side effects of HIT is immediate diagnosis with measuring platelet and the drug is discontinued.