Abstract: Aiming to solving problems existing in previous internal thread inspection method by hand such as large amount of work, an intelligent thread inspection instrument is developed using single chip technology based on eddy current method. First impedance diagram of the probe to the thread state and test frequency is established by designing a simple impedance measuring circuit, and then through analyzing the impedance diagram, suitable amplitude detection frequency range is find out. At last, by adding an appropriate sine excitation signal to the probe and measuring ptobe ends voltoge to complete the thread inspection. Actual test results to a given sample show that the developed thread inspection instrument can accurately judge the thread state of half thread, no thread and full thread, and give a corresponding output signal.