The giant magnetoresistive (GMR) reader nonlinear response in perpendicular magnetic recording should he characterized and studied for the impact on bit-error-rate (BER) performance, because a waveform of perpendicular recording is influenced by GMR nonlinearity differently from longitudinal recording. In this study, the read/write characteristics and BER performance under the influence of GMR nonlinearity were investigated. For a GMR saturation parameter, the amplitude ratio of the fifth harmonic spectrum of isolated patterns to same spectrum of di-bit pattern was measured. This saturation parameter is a good indicator to estimate GMR saturation. GMR asymmetry has a different impact on BER performance from GMR saturation. The raw isolated waveform of perpendicular recording is inappropriate to estimate GMR asymmetry because it does not indicate an accurate dc-ground level with the low-frequency cutoff channel. A specific test patterns which can provide dc-ground level, was found. The measured asymmetry of this test pattern correlates closely with the asymmetry of measured GMR transfer curves. The peak shift of a differential waveform is also a good indicator to estimate GMR asymmetry. Signal processing simulation and some measurement results showed BER performance is influenced by GMR nonlinear response. The impact of GMR nonlinearity on BER depends on their feature. GMR asymmetry has a strong correlation with BER performance.