In this paper, we present a simple and feasible phase-selective synthetic approach to kesterite-phase and wurtzite-phase CZTS nanocrystals by controlling the concentration of sulfur and DDT. It is found that the sulfur precursor has an effect on the crystal phase formation and phase purity of the as-synthesized CZTS nanocrystals (NCs). With the increase of the amount of DDT substituted sulfur, CZTS NCs gradually evolve from kesterite-structure nanoplates to wurtzite-structure nanorods, and the as-synthesized CZTS NCs become smaller and more uniform. Time-dependent phase evolution results suggest that a small amount of DDT restrains the reactivity of the sulfur precursors, phase composition gradually evolves from Cu_1.94S then to (ZnS)_x(Cu₂SnS₃)_1−x intermediate compounds and finally to kesterite CZTS NCs. Conversely, a small amount of S accelerates the decomposition of the DDT thiol group, which results in phase composition transformation from Cu_1.94S then to (SnS)_x(Cu₂ZnS₃)_1−x intermediate compounds and finally to wurtzite CZTS NCs. The present study is a contribution towards understanding the phase-selective formation mechanism of CZTS nanocrystals, and provides a feasible approach to the synthesis of similar materials.