Advanced process control (APC) of photomask dry-etching has been studied for strict mean control of both CD and phase angle of phase shift masks (PSMs). Equations to correlate process information with actual etching results have been developed for this purpose. It is showed that plasma reactance measured with RF sensor has noticeable correlation with Cr etching bias, which is affected by Cr load and condition of etching chamber. Simulation of etching bias based on plasma reactance shows the good agreement with the trend of actual etching results. Expectation of process capability index (Cpk) for mean-to-target (MTT) within 5.2nm is about 1.27, corresponding to CD yield more than 99.9%. In case of MoSi based PSMs, monitoring the sensor outputs is also useful to simulate the etching rate of phase shifter. One simple relationship can be also derived as the case of Cr etching bias. Expected phase error is within 1.5degree in almost cases. In actual photomask fabrication, maintenance of the equation for APC is a critical issue to guarantee the high process yield for a long period. It is showed that trend of the plasma reactance gives the meaningful information effective in automatic maintenance of the equations. As a conclusion, it is proved that our APC method is one of the answers to give the highest MTT yield for both CD and phase angle.