Objectives : The aim of the current study was to investigate the prevalence of sleep bruxism in preschool-aged children in Qazvin, Iran. Method: In a cross-sectional study, using a cluster random sampling method, 400 children aged 3 to 6 years in Qazvin were selected. Diagnosis of sleep bruxism was stablished according to the criteria of the American Academy of Sleep Medicine (AASM) and based on the parents reports. Data collection was done by administering Farsi version of Sleep Habits Questionnaire (CSHQ) and Preschool Behavior Questionnaire (PBQ) to the parents. In addition, dental examination was performed. Data were analysed using chi-square test. Results: Sleep bruxism was identified in 76 children (19%). Sex Ratio (male to female) was 1.5. Sleep bruxism showed significant relationship with positive family history of bruxism (p=0.004), nail bitting (p=0.001), dental decays (p=0.001), attention deficit/hyperactivity disorder (p=0.04), snoring loudly during sleep (p=0.005), sleep talking (p=0.002), excessive limb movements (p=0.02) and restless sleep (p=0.01). Conclusions: The high prevalence of sleep bruxism in preschool-aged children demonstrates the necessity to further research on this issue.