Abstract: A method based on atomic radius and electro-negativity of alloying constituents was proposed to estimate the glass forming ability (GFA) of a bulk metallic glass. The mathematical model of the ratio of radiuses difference to electro-negativity difference and the critical cooling rate Rc was established. The same relationships with a shape of upturned parabola in all five kinds of alloys were obtained with this model. Based on the above results, four different compositions of Zr-Al-Ni-Cu bulk metallic glasses were designed and prepared, and their critical size Z_max, super-cooling liquid region ΔT_x and the reduced glass temperature T_rg were all measured respectively. The results show that the glass forming ability of Zr₅₄Al₁₃Ni₁₅Cu₁₈ is the best in the four alloys and the GFA order of these alloys predicted using the Δd/Δe model is basically consistent with experimental parameters (including Z_max ΔT_x, and T_rg). It is concluded that the prediction using the Δd/Δe model is a reliable method to compare the glass forming ability between different compositions in a same alloying series.