The phenomena in size-dependent optoelectronic and catalytic properties of MoS2 quantum dots make them attractive in fabricating optoelectronic devices and processing water-splitting. However, growing monolayered MoS2 quantum dots with controlled sizes is challenging and has never been realized on a solid-state substrate. Herein, we experimentally demonstrate the size of monolayered MoS2 quantum dots can be thermodynamically manipulated by varying the orientation of the sapphire substrate from c plane, m plane to r plane and the size can be tuned from 10 nm to 2 nm. An apparent shift in the optical absorption and outstanding size-dependent electrocatalytic capability of the MoS2 quantum dots can be featured. The DFT results verify that the size modulation is associated with the interaction strength between the MoS2 quantum dots and the sapphire substrate. This work provides a new benchmark in the growth of size-tunable monolayered MoS2 quantum dots on a solid-state substrate.