The residual impurity gases in the atmospheric rapid thermal processing (RTP) equipment are becoming an important factor in sub-micron ULSI industry. For example, 1% nitrogen in oxygen decreases the RT oxide thickness, and a small amount of moisture or oxygen in nitrogen may strongly affect the RT titanium silicidation. The effective method to reduce the residual gases is to use process gas to purge the chamber. In the present paper the gas purging mechanisms were in-situ investigated in the flow rate range of 10 to 60 slpm using a Quadrapole Residual Gas Analyser (RGA) and gas sensors. The gases for purging studies are N2, O2, He and Ar. It has been found that there are two regimes in the dependence of the residual gas concentration on purging time. Based on the results of systematic experiments, a purging equation, called Pseudo-PST (“perfectly stirred tank) model, has been developed and was used to give the interpretation of the purging process in the atmospheric RTP system. The limitation of the RGA and the oxygen analyzer used to atmospheric RTP system was also discussed.