High power laser diode arrays (LDA) have been used widely. A nondestructive and effective method is desired to evaluate the quality of LDA. The parameter h is the sinkage at the threshold in the electric derivative curve,and the parameter Q is the ratio of the height to the width of the peak sharp in the second optical derivative curve at the threshold in the presented derivative technique. The dependence of the value of h and Q on the uniformity and quality of the laser diode bars is analyzed. By using the equations derived from the equivalent circuits of the bars,the influence of the bar uniformity on the behavior of the value of h is investigated in theory under certain conditions. Compared the compute results with the experiment results,for the same kind devices,it shows that h and Q are both sensitive parameter about the cell uniformity.