Copper antimony sulfide (CAS) is an attractive material with a suitable bandgap for sunlight absorption and a high absorption coefficient in solar cells. There are few works focused on the size-controllable synthesis of CAS nanocrystals, and no work reports on the special character of bandgap tunability until now. Herein, high-quality monodispersed tetrahedrite (Cu12Sb4S13) nanocrystals are synthesized through a novel solution-based method by using thiols as ligands. The as-prepared Cu12Sb4S13 nanocrystals have a narrow size distribution and wide size range from 2 to 16 nm, while their absorption band edges are continuously tunable from 620 to 780 nm. The quantum size effects with bandgap tunability of Cu12Sb4S13 nanocrystals are observed for the first time.