Introduction: There is growing concern about internet addiction and accessing a valid and reliable assessment tool based on DSM diagnostic criteria which can differentiate normal internet users and internet addicts, is very limited. Young internet addiction questionnaire is one of valid available questionnaires. This study was designed to investigate psychometric properties of the mentioned questionnaire. Method: This descriptive cross-sectional survey research was performed on 233 Isfahan university students in 2008. Research units were selected by quota sampling from universities of Isfahan and completed demographic questionnaire, Young Internet Addiction Test and YDQ. A clinical interview based on DSM-IV-TR was performed on all samples. Data were analyzed using factor analysis, internal consistency, Cronbach’s alpha, bisection, test-retest and ROC curve by SPSS 12 . Results: Factor analysis by principal component and Varimax rotation revealed five factors of social problems, effect on performance, lack of control, pathological use of chat rooms and neglecting educational and occupational duties. Content and convergent validity, Internal consistency ( α =0.88), test–retest reliability (r=0.82) and bisection (r=0.72) were determined and all were acceptable, regarding results. Best cut off point for the questionnaire was 46. Conclusion: Young ’s Internet Addiction Test has acceptable psychometric properties and is a valid and reliable tool that may be used in psychological and psychiatric studies in order to screen normal internet users and internet addicts.