A transmission control system for maintaining good positioning performance even if a dynamic characteristic of a transmission is out of a predicted range. A sliding mode controller provided in a shift controller has two degrees of freedom and can independently specify a follow-up characteristic of an actual shift arm position to follow a target position in a shifting direction and a disturbance suppressing characteristic respectively. The sliding mode controller determines a control input to be supplied to a shift motor of a shifting device. A sliding mode controller provided in a selection controller has two degrees of freedom and can independently specify a follow-up characteristic of an actual shift arm position to follow a target position in a selecting direction and a disturbance suppressing characteristic, respectively. This sliding mode controller determines a control input to be supplied to a selection motor of a selecting device.