Two configurations of the phase compensation method are presented to measure the cell gap of a filled twisted nematic liquid crystal display (TN-LCD). The twist angle of the TN-LCD can be any value between 0°and 360°and the pretilt angle can be any practical value. The cell gap measured by this method is the true TN cell gap, namely the thickness of the LC layer which is primarily responsible for the TN optics. Due to this reason, the phase compensation method can also be applied to measure the cell gap of a color TN cell. The measurement uncertainty of this method is within ±1%.