Two isolated pulse models, the Lorentzian-like and the Mixture model, were used to investigate the effect of GMR heads-media with different geometric and magnetic parameters on the readback pulse shape. The matching of these two models with an actual pulse was compared in detail. The dependence of the readback pulse shape of GMR head on the head-media parameters and non-linear distortions was discussed in this paper. When applying these models to evaluate the performance of a recording system, it is necessary to take into account of the difference between the linear superposition of the isolated pulse and the actual readback data pattern. It was suggested to linearize the captured isolated pulse in order to use the model correctly as a useful tool for evaluating the system performance.