Power quality monitoring products are very popular to monitor and control the power quality in recent years. The uncertainty of voltage unbalance is an important parameter designed in these monitoring products. Some standards specify several sinusoidal waveforms to detect the uncertainty of voltage unbalance. But these waveforms seem not enough and some manufactures do not follow the standards to design the detection process. So this paper proposes a method of detecting the supply voltage unbalance accuracy and gives some detection waveforms for uncertainty of voltage unbalance. The simulation results show good agreement with the IEC standards.