The small engine is used under the severe environment of a large temperature change. Furthermore, it is required to enhance the thermal fatigue strength since the usage environment become more severe for satisfying emission regulations and the demands for high engine power. Hypoeutectic aluminum-silicon alloys are usually utilized for the material of the small engine due to their well-balanced properties such as density, castability and mechanical characteristics. In these alloys, the ductility, which strongly affects the thermal fatigue strength, differs depending on the morphology of the eutectic silicon. Herein, we controlled the morphology of the eutectic silicon by the addition of calcium and investigated castability and the mechanical properties. The sample with different calcium amount and cooling rate in cast was prepared. From cross-section images, it was revealed that the morphology of the eutectic silicon became fine with increasing the calcium amount and the cooling rate. The addition of calcium at least 100 ppm resulted in the morphology change of the eutectic silicon in thick parts even where the cooling rate was slower than 1 K/sec. To clarify the effect of the calcium addition on the castability, the amount of dissolved gas, the fluidity and the porosity of the sample were evaluated. There were not any changes in the amount of dissolved gas and the fluidity until the calcium amount reached 200 ppm, while the porosity tended to get small and disperse. The strength property of samples having the fine morphology of the eutectic silicon was evaluated by tension test and thermal fatigue test. Their elongations showed 1.5 times higher than samples having the coarse morphology of the eutectic silicon, and the thermal fatigue strength was enhanced. The addition of calcium contributed effectively to the enhancement for the mechanical properties of hypoeutectic aluminum-silicon alloys.