A variable compression ratio engine that changes the compression ratio may include a piston disposed to be reciprocated in a cylinder by combustion, a crankshaft rotatably disposed at a predetermined distance from the piston, a crank arm with one end connected with the piston and the other end connected with the crankshaft, a connecting rod connected with the crank arm through a crankpin and with the piston through a piston pin and having an oil chamber, an eccentric cam disposed between the crankpin and the connecting rod and being off-center from the crankpin, a variable piston disposed in the oil chamber to be moved by supplied oil, and a link connecting the variable piston with the eccentric cam to rotate the eccentric cam with movement of the variable piston.