Dependence of the negative nonlinear absorption effect on both modulation degree and frequency was investigated in erbium yttrium aluminum garnet crystals. For a modulation frequency of 1 MHz, two peaks, A and B, were observed in the transmitted waveform. With a decreasing modulation degree, peak C which consists of peaks A and B appeared and a reversed-phase waveform was obtained in the transmitted waveform. Peak A decreased as the modulation frequency increased, and peak B was only observed for modulation frequencies above 50 MHz. In the case of a modulation degree under 35%, the transmitted waveform changed to the opposite phase of incident one for modulation frequencies up to 100 MHz. In addition, the negative nonlinear absorption effect was dependent on the product of the composition of erbium and the sample length. It was suggested that the enhanced absorption was due to stimulated emission in three excited states of Er3+.