An apparatus for controlling a transmission of a vehicle includes a controller which is configured to: set a target vehicle speed of an own vehicle; determine whether to permit a kick down of the transmission of the own vehicle based on the target vehicle speed and a current vehicle speed; determine a target shift stage of the transmission of the own vehicle according to a predetermined transmission pattern based on the current vehicle speed and an open value of an accelerator pedal of the own vehicle; and control the transmission to maintain a current shift stage or to kick down the current shift stage to the target shift stage according to whether to permit the kick down determined by the kick down determination controller when the target shift stage is lower than the current shift stage of the own vehicle.