A thermosetting resin composition comprises (A) at least one dicyanamide compound and (B) at least one polyvalent imide having one or more unsaturated bonds and, if necessary (C) at least one polymerizable compound of epoxy compounds, phenolic compounds and triallyl isocyanurate compounds. A thermosetting prepolymer can be obtained by subjecting the composition to a preliminary reaction with heating to the B stage and this prepolymer can give a cured product having excellent heat resistance of class C and flexibility. An exemplary composition containing 60 parts by wt. of 4,4'-dicyanomidodiphenylmethane and 40 parts by wt. of N,N'-4,4'-diphenylmethane bismaleimide dissolved in methyl Cellosolve is reacted at 70.degree. to 110.degree. C. for about 40 minutes to obtain a prepolymer soluble in methyl ethyl ketone. This prepolymer when admixed with 2-ethyl-4-methyl-imidazole and after removal of the methyl cellusolve will form an insoluble and infusible cured product upon heating at temperatures on the order of 170.degree. to 200.degree. C. for about 80 minutes. The resin composition when admixed with a solvent, is useful for impregnation and when used as a powder, it is useful as a molding resin.