An oil cooling system of an air-cooled engine for cooling the oil reserved in a crankcase of the engine comprises a base plate assembled to the crankcase, an overlapping plate assembled to the base plate; an oil filter supported in the overlapping plate; and radiation fins formed on the base plate and the overlapping plate, wherein the base plate includes an inlet port for receiving the engine oil discharged from an oil pump of the engine, an outlet groove for outputting the engine oil and a first passage groove connecting the inlet port and the outlet groove, the overlapping plate includes an inlet groove opposed to the inlet port, an outlet port opposed to the outlet groove and a second passage groove opposed to the first passage groove to form an oil passage together with the first passage groove, and the base plate and the overlapping plate having through-holes where the filter outlet of the oil filter passes through for the connection with the crankcase.