This work was undertaken to systematically investigate optimal RE,Mg and Cu ratios in ZL108 alloy. By the experimental design simplex method, while mischmetal was added into the melted alloy ,contents of magnesium and copper were regulated. Data obtained in tests were treated by microcomputer to calculate regression equations and to draw out iso-grams. the method shows good accuracy and reproducibility. Results show that the optimum microstructure and mechanical properties are achieved with proper RE,Mg and Cu ratios.