Abstract Background: Iran (I.R.) is suffering the growing problem of opiate abuse. In evaluation of the treatment plan for opium dependent patients, a valid and reliable instrument is needed to measure patients' severity of dependence. This study aimed to assess the reliability and validity of the Persian version of the "Substance Dependence Severity Scale" (SDSS). Methods: After translating the English version of SDSS to Persian and then back translating it to English, a structured interview with 200 opium dependent patients was conducted to collect data. Then, reliability (internal consistency and test-retest procedure) and construct validity were tested. Findings: The alpha coefficient was 0.77 and the correlation between test and retest results showed a high correlation coefficient (0.97) which confirmed the reliability. Construct validity was assessed by a cross-check against General Health Questionnaire (GHQ) and the results were in favor of its validity. But the correlation between the amount of substance usage and scores of the 1st and 2nd questions were not within the acceptable range. Conclusion: The results support the reliability of the Persian version of SDSS, but to confirm its validity further studies are needed. Keywords: Substance abuse, questionnaire, psychometrics, Iran.