The objectives are to elucidate homogeneous mechanisms of mercury oxidization in high temperature and study the important factors on mercury oxidization in the homogeneous reaction. The mercury oxidation experiments were carried out by injecting Hg gas with HCl or Cl_2 gas, using an electrically heated horizontal reaction tube. Additionally, chemical equilibrium and chemical kinetics analyses were also conducted for studying the main mercury elementary reaction scheme theoretically. The results show that HCl has little possibility of mercury oxidation. For Cl_2 addition case, however, the mercury oxidization is enhanced more.