In this paper, we investigated effects of aging at 473K on the relationship between microstructure in the vicinity of the grain boundaries and fatigue strength for Al-1.2%Si alloy. Results obtained show the following features. (1) The fatigue strength lowers with increasing aging time, however, when the aging time is more than 18ks at 473 K, the fatigue strength remains almost the same. (2) When the aging time is more than 6ks, grain boundary precipitates with a size greater than several 10 s of nm are observed. (3) When the aging time is 18 ks, an accumulation of dislocations are observed at the grain boundaries and in the vicinity of grain boundary precipitates, and dislocations increase with the number of stress cycles. (4) When the aging time is more than 6ks, the fatigue fracture surface is mainly intergranular. These results suggest that reduction of fatigue strength results from propagation of microcracks which are initiated at the large precipitates on the grain boundaries.