The nature and the type of fillers in composite resin are multifarious. The purpose of this study is to determine the methods of distinguishing the fillers. After the treatments stated below, the surface of polymerized composite resins was observed by means of SEM. 1) The air-inhibited layer on the surface of composite resins was removed by acetone. Observations showed inorganic filler and prepolymerized filler protrusions on the resin surface; submicron filler and microfiller were noticeable. 2) Following the treatment in #1, composite resins were soaked in fluoric acid in order to dissolve any inorganic filler substance. Results displayed pores in the area of the dissolved inorganic filler while a microporous surface was observed on the prepolymerized filler. By this process the type of prepolymerized filler can be classified according to the forms of the contained inorganic filler. 3) The surface of the composite resin was polished with #1200 sand paper. This method shows the difference between the fillers and the matrix resin; however, a clear distinction of the type of filler is difficult to determine. 4) After treatment in #3, the specimen was soaked in fluoric acid. Similar results as treatment in #2 were found; however, microstructure of the syntered filler was easily observed. It is possible to distinguish the filler types of the composite resins by the above mentioned treatments.