Dental examination and interviewing of all 185 pupils of grade 3 and grade 4 at 9-10 years old who have had full first permanent molars and have had full 4 permanent incisors in Phung Chau Primary School, Chuong My, Hanoi causes result: That pupil rate who have early losing primary molars is 22,70 percent. Rate of losing primary molars of each type under tooth position: The highest losing rate of second primary molars in lower jaw: 44,90 percent, the lowest losing rate of the first primary molars in upper jaw: 7,14 percent. The number of pupils that have a part of jaw early losing deciduous molars is 83,33 percent, the number of pupils that have two parts of jaw early losing deciduous molars is 11,90 percent. Early losing deciduous molars are caused by side - efect of tooth decay: 92,86 percent.