Simple multimeter can not measure True RMS of power frequency if there mixed harmonic components. In order to solve this problem, a method is adopted to design a measurement system by using STC Single Chip Microcomputer (SCM). It changes current into voltage by using I/V conversion circuit. Then analog signal turns into digital signal through AD converter. Finally, the Single Chip Microcomputer treats and calculates the data by the RMS algorithm. Sampling frequency of the system is set as 6400sps by Single Chip Microcomputer. This kind of designment uses software instead of hardware and reduces the costs. The realization of the system shows the validation and transportability of the algorithm. The testing data show that this system can measure True RMS of power frequency. The relative full scale error is smaller than 1.284%.