Arsenic(II) sulfide nanocrystals were fabricated by a wet process from its bulk through cluster-mediate transformation. The bulk arsenic(II) sulfide can be easily dissolved in ethylenediamine to form a solution-like cluster (As4S4-NH2C2H4NH2)). The cluster can be transformed to nanocrystals assisted by consumption of ethylenediamine in protic polar solvents with mild thermal treatment. The nanocrystals show size-dependent fluorescence ranging from UV to the blue, two photon fluorescence as well as cytotoxic effects on cancer cells.