Cu2ZnSnS4 (CZTS) is an interesting candidate for thin-film solar cell applications due to its availability and attractive properties (energy band gap of 1.5 eV and high absorption coefficient). However, CZTS solar cell performance is limited because of the defect formation from small grain size, pinholes, rough surface, and negative defects in CZTS such as CuZn. One of the most possible ways to reduce these negative defects, and to increase solar cell performance is by doping with sodium. In this work, CZTS films were synthesized by sol-gel convective deposition technique and Na was doped in CZTS film using 2 different methods: 1) 5, 10, and 15 mg/ml of NaCl solutions were deposited on CZTS thin-film and 2) 5%, 10%, and 15% of NaCl were mixed with CZTS solution before deposited on a substrate. It was found from the results that Na-doped CZTS thin-films obtained from both methods could improve the grain size, enhance film compactness, reduce the grain boundaries and surface roughness. The energy band gap of the CZTS films was in range of 1.5-1.6 eV which was suitable for the solar cell application. Further investigation revealed that the VOC, JSC, and FF of Na-doped CZTS thin-film solar cells were improved accordingly, leading to better solar cell performance. The solar cell efficiency was increased from 0.04% (undoped CZTS thin-film) to 0.18% (15 mg/ml of NaCl solution doped on the CZTS-thin-film).