Un-doped and Na doped CdS thin films were deposited by chemical bath deposition method. The effects of Na doping on structural, optical and electrical properties of CdS thin films and n-CdS/p-Si heterojunction were investigated. Crystal structure of all CdS thin films was cubic with (111) preferred direction. High Na doping detoriated crystal structure and amorphous structure obtained for 3% Na doping. The grain size of thin films was decreased from 6.5 nm to 4.8 nm and surface homogeneity increased with Na doping. All samples had high band gap for CdS (2.42 eV) due to quantum size effect and band gap of the samples was increased 3.65 eV to 3.84 eV as a function of Na content. Na doped CdS samples had higher resistivity and carrier concentration than that of un-doped CdS. Ideality factors of Na doped n-CdS/p-Si heterojunctions were greater than unity due to nanostructered CdS, which indicates that the diode exhibits a nonideal behavior. It was determined that photovoltaic behavior of n-CdS/p-Si prepared with high Na doped CdS dependent on different semiconductor structure (such as depend on function of Na2SiO3 phase) occurred interface of CdS and Si.