This study used quartz lamp and the small disk heaters as the heating sources to investigate the pumping performance of the two-stage Gifford-McMahon (G-M) closed-loop gaseous helium cryopump with heat transfer effect. In the experiment, the heat was applied by two means, for the first-stage array by thermal radiation and for the second-stage array by applying directly. The test system was constructed in accordance to the single-dome flow-meter pumping measuring apparatus recommended by Welch et al. [J. Vac. Sci. Technol. A 17, 3081 (1999)]. The experiment applied varied levels of electrical power to the heaters, introduced Ar to the test dome, and measured the corresponding dome pressure, gas throughput, and temperatures of the different arrays inside the cryopump. The results showed that the temperature of the introduced gas heated by the heater in the test dome would influence the measured pumping speed of cryopump. When the thermal load is applied primarily on the first-stage array, the temperature of the second-stage array varies significantly as the temperature of the first-stage array changes; when applied directly to the second-stage array, the temperature of the first-stage array varies little as the temperature of the second-stage array changes. The pumping speed of cryopump, and temperature of the two-stage arrays increase gradually as gas throughput rises, and sharply when the gas in the test dome is nonmolecular flow. The measured refrigeration capacity of second-stage cooling station is approximate to the data provided by ULVAC. (c) 2006 American Vacuum Society.