DSPs make it possible to realize the real time DFT calculation for the ac voltage phase detection with sufficiently short sampling period. This paper discusses about the DFT phase detection error caused by the off nominal ac frequency, proposes the error correction scheme using the estimated ac frequency and evaluates the accuracy of corrected phase. The frequency is calculated from the phase difference obtained from DFT, and assesses the estimated frequency error caused by the detected phase error. The presented scheme can suitably correct the phase detection error with choosing the adequate phase difference interval for frequency calculation. This paper also proposes error correction scheme for the positive sequence phase detection of the three-phase system with compensating the harmful influence of the negative sequence component to the highly unbalanced condition. The suitability and the accuracy of the presenting error correction scheme are confirmed through computer simulations with using ATP-MODELS.