Monodispersed copper sulfide nanocrystals were synthesized via the decomposition of an air-stable precursor, copper (I) thiobenzoate (CuTB), in the presence of dodecanethiol (DDT). We discover that, by varying the stabilizing agent used (trioctylphosphine or tributylphosphite), we can selectively generate phase-pure roxbyite nanoplates (Cu 1.75 S) or chalcocite faceted nanocrystals (Cu 2 S). We also demonstrate that, under suitable conditions, the roxbyite nanoplates can be forced to grow only in two dimensions, with an aspect ratio (diameter/thickness) tunable between 2.3 and 4.1. To the best of our knowledge, this is the first report for making nanocrystalline copper sulfide with such aspect ratio tunability. Temperature and the [DDT]/[CuTB] ratio were identified to be the important factors for controlling the size of the nanocrystals. To gain more insight to the mechanisms of phase-selective control, we attempted a series of controlled experiments and DFT calculations. It appears that the precursor can undergo two competitive pathways, leading to seeds, and thus the growth, of different crystal phases. This work thus demonstrates a general approach to phase-selective nanocrystals engineering whereby the kinetics of decomposition of a chosen precursor is readily manipulated using activating or stabilizing agents.