Abstract: In this paper, the principle and realization method of communication between PLC and frequency-converter with Modbus communication protocol were discussed. The speed control method and the communication protocol of frequency-converter and PLC non-sequence communication protocol were introduced. With the example of control to YASKAWA Varispeed G7 by MITSUBISHI Q series PLC and its communication module, it studied how to implement the communication between PLC and frequency-converter by serial communication protocol.