Disclosed herein is a control apparatus and method for improving fuel efficiency in a CACC system, which can improve fuel efficiency through control of a vehicle speed so that a vehicle travels using an optimized cost in consideration of a target vehicle speed, current vehicle speed, minimum driving speed set in the vehicle, and a deceleration distance if the vehicle that uses the CACC system senses a forward vehicle and enters into a CACC active mode. The control method for improving fuel efficiency in a CACC system includes setting a target speed profile based on a target speed of the subject vehicle and an expected driving path, determining whether a target vehicle to be followed by the subject vehicle exists, and controlling the driving speed of the subject vehicle according to the set target speed profile depending on whether or not the target vehicle exists.