The cost of solar-grade silicon cannot be reduced in the Siemens process, which is the major process of producing polycrystalline silicon through current technologies. Therefore, the zinc reduction process, which can reduce solar-grade silicon in a cost effective manner, should be redeveloped for these conditions. The thermodynamics in the zinc reduction process are studied in this paper. When the reactions reach equilibrium, the equilibrium gas phase composition has been analyzed within the temperature range of 1184 to 1500 K at n(SiCl4)/n(Zn) of 1/2, 1/4, 1/8 and P = 1 atm, 3 atm and 6 atm. Furthermore, the relations of the production ratio of silicon and pressure, temperature and the feed molar ratio (n(Zn)/n(SiCl4)) are investigated and the graphs thereof are plotted. The major side-reactions affecting the production ratio of silicon have been established as: SiCl(4)(g) + Zn(g)-> SiCl(2)(g) + ZnCl(2)(g) and SiCl(4)(g) + Si(s) -> SiCl(2)(g). Finally, suitable operational conditions in the practical process of poly-silicon manufacture have been established, with 1200 K, 2 atm and the feed molar ratio (n(Zn)/n(SiCl4)) of 4 at the entrance is the best conditions. A production ratio of silicon of 90.3% is achieved under these conditions. Furthermore, 1184 K, 2 atm and the feed molar ratio (n(Zn)/n(SiCl4)) of 4 are the ideal conditions. Under these conditions, the theoretical yield is 98%.