Friction welding of 2017 and 6061 aluminum alloys in as-received, solution heat treated and aged states was conducted under the optimum welding condition for as-received specimens. As friction-welded, tensile strength ofjoints was approximately equal to the mother 6061 alloy for every temper states. When the joints were post-heat-treated, brittle fracture occurred at the welding interface and/or pan-cake-like recrystallized boundaries in the vicinity of the interface. A mechanism of friction welding and an origin of embrittlement were discussed on the basis of the effects of heat treatments, on the hardness distribution changes of the joints and also on the fractographical features of tensile specimens.