A method of controlling an engine speed of a hybrid vehicle including a motor that is connected to a transmission, a battery that provides a driving voltage to the motor, an engine that is selectively connected to the motor through an engine clutch, and a hybrid starter-generator (HSG) that is connected to the engine includes: comparing, when the engine is started, an external air temperature to a setting temperature; setting a battery rated derating factor based on the comparison of the external air temperature to the setting temperature; determining an output area of the HSG according to a battery voltage; and outputting an available torque of the HSG based on the set battery rated derating factor and the determined HSG output area.