Objectives: Given the relatively high prevalence and emphasis on the pathogenicity of Blastocystis sp., especially in immunocompromised patients and other at risked populations like military personnel, this study was carried out to investigate the prevalence of Blastocystis sp. in the city of Karaj during 2014-2015. Methods: In this descriptive study, 2500 stool samples were screened for Blastocystis sp. using microscopically examinations and iodine-stained, wet-mount preparations and formalin-ether as a sedimentation method. Demographic information such as age, gender, and symptoms of patients, were recorded in the questionnaire. Three stool samples were collected from each patient in disposable plastic containers. Data were analyzed using chi-square, ANOVA, and t-test SPSS V. 24 software. Results: From the total of 2500 cases, 345 (13.8%) cases were positive for Blastocystis sp., 56.5% of patients were male, and 43.5% were female. A total of 33.3% of patients had a clinical symptom and 66.7% of patient had no clinical symptoms. Anorexia was the most common symptom in patients with Blastocystis sp. Conclusions: The use of appropriate diagnostic laboratory procedures in hospitals and medical centers can provide an effective and accurate diagnosis that will play an important role in the promotion of community health; considering some unspecific signs could help the health professionals to have correct diagnose.