The structures and mechanical property of the unidirectionally solidified Al-Co eutectic alloy were investigated. Al-Co eutectic alloy was unidirectionally solidified with growth rates(R) between I cm/hr and 10 cm/hr in the induction furnace maintaining the thermal gradient (G) at solid-liquid interface, $32^{circ}C$/cm. The eutectic microstructure was varied with the growth condition(G/R ratio). When the G/R ratio was larger than $8.5{ imes}10^{3}^{circ}C/cm^{2}/sec$, a lamellar structure was formed, But the G/R ratio was smaller than $8.5{ imes}10^{3}^{circ}C/cm^{2}/sec$, a colony structure was formed. It was found that the interlamellar spacing(${lambda}$) was dependent on the growth rate(R) with the relationship, ${lambda}^{2}{cdot}R=constant$. The microhardness of this eutectic alloy increased with increase in the growth rate.