Abstract: A crankcase scavenging mechanism for a four-stroke engine is disclosed. A valve operating chamber is assembled from a cylinder head and a cylinder head cover and is communicated with a crankcase. The cylinder head cover is arranged on the top of the cylinder head and is provided with a gas-oil separating cavity on the top thereof. A passage is communicated with the valve operating chamber. The gas-oil separating cavity is connected with a partition plate to form an oil-gas separation chamber. The oil-gas separation chamber is provided with a one-way valve thereon. The one-way valve is installed in a one-way valve hole. A passage above the one-way valve is provided with an inclined orifice, one end of which is blocked by a plug, the other end of which is communicated with the cylinder head and is further communicated with a suction tube through a cylinder head communication passage. An oil-returning hole is formed between the inclined orifice of the passage and the oil-gas separation chamber.