Objective: The aim of present study was to evaluate the prevalance and causes of premature loss of deciduous molar of premature loss of primary molar of childrens 7-10 years at Tan Mai primary school - Hoang Mai - Hanoi. Describe the effect of premature loss of primary molar on the dental archs, from that estimated the eruption of premolar. Methods: Cross-sectional descriptive study. These childrens with premature loss of deciduous molar were selected from 711 children school from 7 to 10 years of age. Study casts were examinated and taked alginate impression. Evaluated the space loss, relationship first molars, rotation, crowding and unerupted of other teeth. In case with space loss, taking panoramic radiographs to prognosis for premolar eruption. Results: The prevalence of premature loss of primary molar: 20.96 percent. Mainly causes premature loss of primary molars was decay (95.97 percent). Changing of pace in premature loss side measured by two primary molars space (12.93 + or - 2.34 mm on maxillary
13.90 + or - 2.44mm on mandibular) was significant less than the side without loss teeth (15.08 + or - 1.55mm on mandibular). 22.15 percent premolar erupted crowding, unerupted (second premolar was more than first premolar). 63. 24 percent: lack of space for premolars eruption. Conclusion: Premature loss of deciduous molar cause space deficiency, malocclusion, crowding teeth.