In this paper, the authors study the basic principle of automated test case generator for c language. The paper proposes a design principle to convert c source code to control flow graph, and then by using this control flow graph they find out all possible feasible paths and generate test case automatically by using CFG and random number testing technique. They know that software testing is one of the important aspects of software development process. They also know that c is a general purpose programming language, widely used for portable application and system programs is very important language and covers much critical application.