The effect of heat treatment on the microstrueture and Superconducting properties of the eutectic alloy Al- 11.3 at % Si prepared by the splat quenching technique has! been studied. The results of structure analyses indicated that the rapidly cooled Al-11.3 at % Si alloy consists of two phases: a supersaturated α-Al(Si) solid solution and an amorphous phase of Al and Si atoms which are distributed statistically. After annealing at 100℃ for 50 hr. the Si content of the solid solution phase is reduced and some dispersive Si granules separate out from the amorphous phase. After annealing at 200℃ for 100 hr. the specimens precipitated completely. Superconducting transition temperature Tc was measured. The anomalous magnetoresistivity in supereondueting-normal transitions has been found in the sample after annealing at 100℃ for 50hr. Our results reveal the existence of two superconducting phases in this sample.