Abstract：The leaching pretreatment of refractory gold ores by bacteria has advantages of lower operating costs, less environmental pollution and higher efficiency of treatment. So it is considered to be a promising effective method for pretreatment of refractory gold ores. In this study, samples were taken from the Huashiyou gold deposit, Zunhua, Hebei. The samples are refractory ore, a kind of pyrite containing gold. Several strains of Thiobacillus ferrooxidans were used in pretreatment of the samples. Among those, D3 strain was found to decompose sulfide ores effectively. If the size of the ore is 200-mesh, the cyanidation rate of gold rises from 63.9 (without biooxidation ) to 88.2% (after biooxidation). For gold ore with a grain size < 100 mesh, the cyanidation rate of gold may also generally rises >10%. In addition, some conditional experiments for the process of treatment of gold ore by this strain were also performed; for example, the size influence, pH change and strengthening of the physical method were studied. Some rules in the biooxidation process of gold ore and the optimum controlling conditions during practical operations were obtained from the experimental results.