A variable compression ratio apparatus may include: a connecting rod having an end connected with a crank pin of a crank shaft; a plunger housing formed at the other end of the connecting rod, the plunger housing having a space to which a plunger is inserted. In particular, the plunger moves upwardly and downwardly, and an outer circumference surface of the plunger contacts an inner circumference surface of the plunger housing. The apparatus further includes a protrusion protruded from the plunger to the plunger housing and connected with a piston, and a hydraulic pressure control valve for supplying hydraulic pressure to the space and moving the plunger by supplying hydraulic pressure.