We investigated the growth condition of high-quality Cu2ZnSnS4 (CZTS) single crystal by traveling heater method (THM) which is one of the solution-based growth methods. We selected Sn as solvent because it is a constituent element of CZTS with low melting point. The solubility of CZTS in Sn solvent was approximately 0.01 mol% at 600 °C and 0.28 mol% at 1000 °C. It is found that CZTS is soluble in Sn solvent, which is similar to CuGaS2 in In solvent. The Sn solutions saturated with CZTS solutes less than 60 mol% were separated with two phases, less than 30 mol% solutions were CZTS and Sn phases and solutions from 30 to 60 mol% were CZTS and SnSx compound phases. It is impossible to grow CZTS single crystal from less than 60 mol% Sn solutions, because secondary phases located at the bottom of the ampoule and interfered with single crystal growth. Optimum growth condition of CZTS was determined based on CZTS–Sn phase diagram, which was growth temperature 900 °C and X = 70 mol% solution. The electrical properties of CZTS single crystal were observed along the growth direction and it can be clearly seen that the electrical properties are homogeneous along the growth direction.