Solidification of Zn-5wt%Al alloy was investigated by DTA and quantitative metallography analysis. The effect of cooling rate on eutectic lamellar spacing was studied. Based on analysis of heat transfer during solidification, relation between cooling rate and eutectic lamellar was derived. Curve fitting was carried out according to the data of experiment. The results indicate that eutectic lamellar spacing is approximately inverse ratio with the one fourth power of cooling rate.