The Sangdong Tungsten mine in Korea produces molybdenite and bismuthinite as by products. The flotation of the above minerals has been carried out after differential oxidation of bismuthinite and molybdenite surfaces by primary roasting, though there is no theoretical view points to the above process. In this experiment, the oxidation degrees of the ore surfaces were measured at the various temperatures by thermobalance, and the flotation was carried out with the samples which were roasted at the different conditions. During the roasting molybdenite surface was also oxidized as much as bismuthinite at low temperature. The flotation recovery of the molybdenite which was roasted above 250℃ was rapidly decreased. It seems to be difficult to say that depression of bismuthinite was caused by differential surface oxidation.