Abstract: When Nyquist criterion is used to judge the system stability, two conditions must be provided: the open-loop amplitude-phase characteric curve of the system, and the number of poles on the right half of the S plan of the open-loop transfer function. It is presented in this paper that how to utilize the mapping theorem to determine the number of open-loop poles on the right half of the S plan while drawing the open-loop amplitud-phase characteristic curve of the system.