The aim of this study was to determine prevalence of anterior open bite in 12-15 years old students in Zahedan.The samples were 10375 students who selected from secondary school of Zahedan by multi stage random sampling method (5110 females ,5265 males).All samples had permanent dentition and had no history of orthodontic treatment ,truma or surgery that could affected occlusion and no extraction .They examined under natural light by disposable mirror and anterior open bite were measured by a disposable plastic ruler in centric occlusion.The data were recorded in a questionear for each sample.The data analysed through the chi-sqare test with level of significance of 5%.The result showed that the prevalence of anterior open bite was 1.6%(1.9% females,1.3% males)and about 70% of these cases had mouth breathing and after it tongue thrusting was the most prevalent habits.According to these results ,we may conclude that mouth breathing is important factor in prevalence of anterior open bite.