In this letter, we propose a 2-speed small transmission mechanism based on twisted string actuation (TSA). The proposed mechanism achieves a compact and simplified two-stage transmission ratio by adjusting the radius of the twisted string. This transmission mechanism does not require the complex mechanical parts and mechanisms of conventional transmission systems. In addition, the mechanism can achieve a braking effect without an additional mechanical component via the nonlinearity of the rotational-linear transmission provided by TSA. The prototype is sufficiently small to be embedded in a robot hand and can be easily installed in a small robotic system. This mechanism can provide an energy-efficient driving strategy and a wider driving range for small robot systems.