In this paper, we presented a computational modeling of frequency relay for islanding detection of distributed generation (DG) by the performance of frequency estimation methods. For this end, are presented an analysis and application of four methods of fundamental frequency estimation applied in distribution systems with DG in islanding situation such as: (a) Zero crossing, (b) adjustment of points to a sinusoidal waveform (APSW), (c) hybrid method, which combines (a) and (b), and (d) DFT algorithm. For the evaluation and validation of the performance of the frequency estimation methods were carried out by using the performance curves. The simulations and results prove the effectiveness of frequency methods for be used in the protection for islanding detection.