Abstract: A model of risk judgment and avoidance for the software test automation,based on the Return On Investment (ROI) model and COCOMO measurement results,is presented. The maintenance cost of regression test is also taken into account. The model first proposes a concept of mean maintenance cost factor and calculates its threshold,and then analyzes the each time maintenance cost factor by normal method. According to normal distribution,we can obtain the possibility for the test automation risk occurrence. At last a strategy for the risk avoidance is proposed by the sensitivity analysis. The model provides dynamic guidance and correct method for the whole software test automation in order to avoid risk occurrence.

Key words: test automation cost, mean maintenance cost factor threshold, each time maintenance cost factor, risk avoidance