The effect of prior T4 heat treatment and subsequent microstructural evolution on tensile behavior of friction stir welded (FSWed) AZ31B-H24 alloy was examined in this study. Selected AZ31B-H24 plates were prior T4 heat treated at 400 for 24 hours and subsequently friction stir welded. The tensile properties, optical micrographs and SEM fractographs for FSWed AZ31B-T4 specimens were compared with those for the H24 counterparts. Prior T4 heat treatment tended to decrease the tensile ductility reduction in FSWed AZ31B-H24 specimen. The tensile ductility reduction mechanism as associated with prior T4 heat treatment is discussed based on detailed micrographic and fractographic observations.