A variable compression ratio engine includes a variable chamber housing in fluidic communication with a combustion chamber of the engine, a chamber plunger slidably disposed within the variable chamber housing and forming a variable chamber together with the variable chamber housing, a hydraulic pressure cylinder connected with the variable chamber housing, a hydraulic piston slidably disposed within the hydraulic pressure cylinder, having a slider protruded to form first and second operation chambers with the hydraulic pressure cylinder and connected with the chamber plunger, a control plunger connected with the hydraulic piston, a control cylinder for receiving the control plunger and slidable with respect to the control plunger, a compression ratio control portion selectively moving the control cylinder along the length direction, and hydraulic pressure lines supplying control hydraulic pressure to the first or second operation chamber or releasing the control hydraulic pressure from the first or second operation chamber according to relative positions of the control cylinder.