A transmission control system, which is adapted to be installed in a vehicle body of a two-wheeler, includes a transmission assembly, a mobile device and a transmission controller. The transmission assembly is configured to be disposed on the vehicle body and to perform a gear-shifting operation. The mobile device is configured to be detachably disposed on the vehicle body. The mobile device includes an orientation sensor capable of detecting a current center-of-gravity of the vehicle body to generate a current center-of-gravity datum. The transmission controller is configured to be disposed on vehicle body, to receive the current center-of-gravity datum, to calculate a roll angle variation according to the current center-of-gravity datum, and to output a gear-shift control command according to the roll angle variation. The transmission controller is configured to indicate the transmission assembly whether to perform the gear-shifting operation according to the gear-shift control command.