The present disclosure provides a method of diagnosing an oil leak of an engine clutch in a hybrid electric vehicle including: when the engine starts up in the hybrid electric vehicle, operating a hydraulic actuator provided in the engine clutch, wherein the hydraulic actuator is configured to perform engagement or disengagement between an engine and a motor; determining displacement of a piston provided in the hydraulic actuator and pressure for any displacement of a plurality of the displacements of the piston; calculating a pressure gradient based on the displacement of the piston and the pressure for any displacement of the plurality of the displacements of the piston; and determining the oil leak of the engine clutch based on the pressure gradient.