Abstract: The authors proposed a new comprehensive evaluating index the time-dependent stability of tool wear in cutting tool performance judgments . Two examples are given based on two groups of short-time cutting tests.The Conditional mean-value and variances of tool-life or tool-wear are accordingly es timated , which enabled the overall comparison of cuttiug performances of two cemented carbide grades. Discussion and modification are also made on the understanding of the "first period of tool wear process".