An apparatus and a method for controlling a transmission of a vehicle may include a determining device configured to determine whether a curved road is present within a predetermined distance in front of the vehicle, based on information on a front road, a calculating device configured to correct, based on information on a gradient of the curved road, a vehicle speed in starting cornering of the vehicle on the curved road, determine an expected lateral acceleration based on the corrected vehicle speed and information on a curvature of the curved road, and determine a pattern correcting coefficient based on the determined expected lateral acceleration, a pattern correcting device configured to correct a gearshifting pattern of the transmission, which is preset, based on the pattern correcting coefficient, and a control device connected to the determining device, the calculating device and the pattern correcting device and configured to control the transmission based on the corrected gearshifting pattern when the vehicle enters the curved road.