This study reports the effects of H2S gas concentration on the properties of Cu2ZnSnS4(CZTS) thin films. Specifically, sulfurization process with low H2S concentrations of 0.05% and 0.1%, along with 5% H2S gas, was studied. CZTS films were directly synthesized on Mo/Si substrates by chemical bath deposition method using copper sulfate, zinc sulfate heptahydrate, tin chloride dihydrate, and sodium thiosulfate pentahydrate. Smooth CZTS films were grown on substrates at optimized chemical bath deposition condition. The CZTS films sulfurized at low H2S concentrations of 0.05 % and 0.1% showed very rough and porous film morphology, whereas the film sulfurized at 5% H2S yielded a very smooth and dense film morphology. The CZTS films were fully crystallized in kesterite crystal form when they were sulfurized at 500 oC for 1 h. The kesterite CZTS film showed a reasonably good room-temperature photoluminescence spectrum that peaked in a range of 1.4 eV to 1.5 eV, consistent with the optimal bandgap for CZTS solar cell applications.