Step height is an important parameter of micro-electronic mask. A new approach named dual-frequency interferometric confocal microscope (DICM) is proposed which combines scanning confocal microscopy with heterodyne interferometry. The phase of heterodyne interferometry is recorded as an aim criteria when optical intensity of confocal microscopy is in maximum. This method implements high resolution (0.1 nm) and relatively large measurement range (>15 μm) simultaneously. Also the step height measurement range depending on the range of Z axis PI-Foc scanner which theoretically can be extended to 100 μm. The results show that in ordinary laboratory condition, with temperature controlled , the drift of phase measurement is about 5 nm in an hour. The 20 μm step height has been successfully measured and the results agree with the actual value.