To provide a silicon resin compsn. which can give a molded item excellent in corrosion resistance and durability even in air at high temps. by treating a mixture of a polysilane an unsatd. carbon compd. at a specified temp. or higher.

This resin compsn. is obtd. by thermally treating a mixture of 1 pt.wt. polysilane and 0.05-100 pts.wt. usatd. carbon compd. at a temp. 220°C or higher, at which Si-Si bonds in the polysilane are cleaved and C atoms of side chains are inserted between Si atoms of the main chain to form Si-C-Si bonds. The polysilane is a linear or cyclic polysilane of the formula: (R12Si)m or a network polymer having an Si-Si backbone represented by the formula: (R2Si)n or (R32Si)x(R3Si)ySiz, [R1 and R2 are each H, alkyl, alkenyl, hydroxyl, or the like; R3 is H, alkyl, alkenyl or the like; (m) is 2-10,000; (n) is 4-10,000; and x+y+z is 5-10,000]. This compsn. can give a molded item excellent esp. in heat cycle characteristics.