Cu2ZnSnS4 (CZTS) holds great promises as an absorber material for sustainable and low cost thin film solar cells. Kesterite and wurtzite are two common phases of CZTS. Until now, the synthesis and the growth of both phases are not clearly understood. In this work, kesterite CZTS nanoparticles, wurtzite CZTS nanoparticles as well as CZTS particles with a mixture of both structures were prepared by using elemental sulfur, 1-dodecanethiol, and thioacetamide, respectively. Time dependent studies were conducted and the reaction rate of sulfur source was found to be the key factor in determining the phase formation. Elemental sulfur reacts with oleylamine to produce highly reactive small molecule H2S, which leads to the formation of kesterite phase. The reaction pathways of the long alkane chain 1-dodecanethiol yield the formation of wurtzite phase via a binary phase. Thioacetamide yields a mixture of kesterite and wurtzite phase in the final product. The optical and electrical properties of kesterite and wurtzite CZTS were also evaluated.