Abstract: Objective: To explore the clinical effect of dental filling materials commonly used in pediatric dentistry on the restoration of mesial or distal occlusal surface cavities (class Ⅱ cavity) in primary molars. Methods: 266 primary molars with class Ⅱ cavities among 178 children aged 4-6 were randomly divided into two groups, namely, the 3M flowable resin group (experimental group) and 3M glass ionomer group (matched group). The clinical effect of two materials was compared after 1 and 2 years of treatment. Results: Treatment success rates of 3M flowable composite resin and 3M glass ionomer for class Ⅱ cavities were 96.00% and 87.80% respectively after 1 year. After 2 years, the rates were 95.76% and 86.61% respectively (P<0.05). Conclusions: For class Ⅱ cavity in primary molars, the clinical outcome of 3M flowable composite resin is better than that of 3M glass ionomer.