A write assist circuit capable of writing data to a memory cell with a bit line and a bit line bar is provided. The write assist circuit includes a clamping circuit, and first and second sense amplifiers. The clamping circuit is coupled to first and second nodes to prevent the voltage of the first and second nodes from being lower than a data-retention voltage. The first and second nodes are supplied with first and second voltage sources. The first and second sense amplifier are utilized to detect the voltage of the bit line or the bit line bar, amplify the voltage and pull down the voltage of one of the first or second node according to the data while the voltage of the other one of the first or second node is kept at a power supply voltage level.