An aspect of the disclosure provides an apparatus and a method for improving fuel efficiency of a plug-in hybrid electric vehicle. The apparatus for assisting driving of a host vehicle includes an input configured to receive an input for activation of a fuel efficiency mode; and a controller configured to: in response to receiving the input for activation of the fuel efficiency mode, determine a control factor for power distribution based on route information received from a navigation device of the host vehicle and a state of a battery, and perform power distribution based on the control factor.