The compressibility of liquids was measured in a gasketed diamond anvil cell using a modified piston displacement method. A pair of metal markers was fixed on the diamonds. The gasket thickness at different pressures was measured by monitoring the distance change between the markers using a micrometer microscope. The area of the gasket hole was measured by means of microphotography. Then the product of the average area and thickness of the gasket hole determined the sample volume. Using this method we obtained the compressibility of a 4:1 v/v methanol-ethanol liquid mixture at hydrostatic pressures up to 8 GPa and found that the liquid volume at 8 GPa decreases to 51.5% of the initial volume. By the least-squares method, we fitted the experimental data to the Tait equation and obtained C=0.4055, B=0.5436 GPa. The relative error is estimated to be about 1.7%. This simple and convenient method could significantly extend the pressure range of liquid compressibility measurements.