An apparatus for improving cooling performance of an engine room in a vehicle, may include an air duct connected to an air cleaner to serve as a moving guide of fresh air; a duct flap installed to open and close some section of the air duct and operated to supply all the fresh air introduced into the air duct when the air duct is in a closed state to the air cleaner and discharge some of the fresh air introduced into the air duct when the air duct is in an opened state to a space of a high temperature part of the engine room; and an actuator supplying power to the duct flap to perform an opening and closing operation of the duct flap.