The thermal degradation and the combustion tests of epoxy resins (prepolymer) whose molecular weights are from 1,650 to 50,000 have been conducted. From the result of the vertical firing experiments of epoxy resins with different molecular weight, it was found that there is a molecular weight region where combustion do not continued because volatile gas components are hardly generated by burning. In the thermo-gravimetric analyses, it was observed that the degradation behaviors did not change for the difference of the molecular weight. In the relation between the molecular weight and combustibility, it was observed that the molecular weight distribution did not change in the samples that did not burn. On the other hand, it was seen that molecular weight lowered in the burned sample. These results suggest that there is a possibility that the combustibility can be controlled by adjusting the molecular weight.