A method for making metal sulfide nanocrystals is provided. First, some metal salt powders are provided in a container. Second, superfluous mercaptan is added into the container. Then the superfluous mercaptan and the metal salt are stirred uniformly to get a precursor mixture and the precursor mixture is heated to get the metal sulfide nanocrystals. Lastly, the metal sulfide nanocrystals are separated and washed.