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 1 cm/hr and 10 cm/hr in the induction furnace maintaining the thermal gradient (G) at solid-liquid interface, 32℃/cm. The eutectic microstructure was varied with the growth condition(GIR ratio). When the G/R ratio was larger than 8.5×10³℃/cm²/sec, a lamellar structure was formed, But the GIR ratio was smaller than 8.5 ×10³℃/cm²/sec, a colony structure was formed. It was found that the interlamellar spacing(λ) was dependent on the growth rate(R) with the relationship, λ²·R = constant. The microhardness of this eutectic alloy increased with increase in the growth rate