The some characteristics of the underwater friction welded joints of 6061 aluminum alloy, which were made by using a brake type friction welding machine, were examined with different welding conditions. The highest temperature of the friction welding process on the underwater welding was considerably lower than that of the atmospheric welding, and both friction pressure and friction time did not hardly affected to the temperature on the friction welding process. The total loss of the underwater welded joint became smaller compared with atmospheric welded joint, moreover the width of softening area on the underwater welded joint reduced and the softening ratio also decreased. The maximum joint efficiency of the underwater welded joint could be obtained 86%, it was slightly lager than that of the atmospheric welded joint which was 82%. The elongation of the underwater welded joints were lower than that of the atmospheric welded joint. Both the underwater and atmospheric welded joints fractured in the softened area under the tensile load.