Copper selenide (Cu3Se2) nanocrystals were successfully synthesized on glass substrate by the chemical bath deposition technique. The room temperature formation of Cu3Se2 from the reaction between Cu-ions and selenosulfate in aqueous solution was investigated. The resulting precipitate from the reaction of Cu-ions and selenosulfate was initially light green (mainly a mixture of amorphous Cu2SO3.CuSO3 and Cu(OH)2) and turned to greenish brown and finally brownish (mainly Cu3Se2). Cu3Se2 was formed via the hydroxide cluster mechanism and reaction between Cu(OH)2 and the slowly generated Se2- ions. The Cu3Se2 exhibited a disordered phase and small nanocrystals in the range of 2 to 5 nm. The formation mechanism of different phases of copper selenide by chemical bath deposition technique were explained in details.