Cu2ZnSnS4 (CZTS) precursor was fabricated from a CZTS nano crystal ink, which was obtained by a ball-milling method. The precursor was then annealed in the sulfur atmosphere. The growth mechanism of CZTS was studied in this work. It was found that with the extension of annealing time, the grain size as well as the crystallinity was improved significantly. And growth of grains from domain area of CZTS nano particles was observed. The samples experienced a decomposition process during the annealing. Secondary phases, such as ZnS, SnS, CuS and CuSnS3, were detected at the beginning of annealing process. However, with the increase of annealing time, the secondary phases recombined to form uniform CZTS film, which lead to the growth of grain size and improvement of crystallinity. The growth mechanism of CZTS films during the annealing process was concluded as: CZTS(Nanoparticle)→(Cu2S, ZnS, SnxS)→CZTS (large grains).