A source driver and a method for determining polarity of pixel voltage thereof are provided. The source driver includes a data register unit, a plurality of data groups and a plurality of polarity determining units. The data register unit receives an image data signal and provides a plurality of display data. The data groups have at least two data channels respectively. The data channels are coupled to the data register unit to receive the corresponding display data and provide a plurality of pixel voltages. The polarity determining units are respectively coupled to the data channels corresponding to different data groups, and each of the polarity determining units determines whether to invert polarities of a part of the pixel voltages provided by the coupled data channels according to the received display data of the coupled data channels and previous display data corresponding to the received display data.