Automatic generation of test data for a given path in a program is one of the elementary problems in software testing, the difficulty of which lies in the fact that how to solving nonlinear constraint is unsolvable in theory. Gupta et al proposed a method, which is referred to as the Iterative Relaxation Method, to address the above problem by linearizing the predicate functions [1 ]. This paper improves the Iterative Relaxation Method by omitting the constructions of predicate slice and input dependency set, and proves the equivalence of systems of constraints generated by both methods. Since it is not needed for our method to analyze the dependencies between statements on the path in the course of deriving of system of constraints, our method still work when some statements are only object or executable codes rather than source codes on the path. Therefore, our method is fit for generating test data for black-box testing and regression testing also. We have developed a prototype of path-wise test data generator whose fundamental algorithm is presented in this paper. The initial experiments with this prototype have shown that our method is practical.