Copper Indium diselenide (CIS) nanocrystals were synthesized by a new room-temperature reverse microemuslion route. By changing the water to surfactant ratio, the size of the particles can be readily adjusted while the composition of the particles can be tuned by changing the precursor ratio. The UV-visible absorption and photoluminescence spectra show blue shift for CIS particles with smaller size. The corresponding band gaps of the as-prepared CIS particles are approximately 2 eV, which is larger than that of bulk CIS.