A variable compression ratio engine may include a variable chamber housing communicated with a combustion chamber of the engine, a variable chamber piston slidably disposed within the variable chamber housing, forming a variable chamber communicated with the combustion chamber within the variable chamber housing, forming a hydraulic pressure chamber between the variable chamber piston and the variable chamber housing, and having a leaking line for flowing oil leaked from the hydraulic pressure chamber, a connecting shaft connected to the variable chamber piston, and having an oil drain line for exhausting the oil in the leaking line is formed thereto, an oil supplier supplying oil to the hydraulic pressure chamber, and a compression ratio controller connected to the connecting shaft and controlling a relative position of the variable chamber piston.