A comparative study of the Sigma Delta Modulation(SDM)and the Pulse Code Modulation(PCM) is presented, and the advantages and the disadvantages of the two kinds of modulation are also obtained. A 2-bit SDM simulated system and a 2-bit (PCM) simulated system for a micro-accelerometer are established in this paper. Then, the accelaration signals pass through the modulations are comparatively analyzed, and the difference of two mudulations output waves is obtained by the comparive analysis, Finally, the power spectral density patterns of two kinds of the modulated signals are com-puted with a periodogram respectively. The power spectral density from experimental results indicates that the quantization noise of the PCM is more serious than that of the SDM in the low frquency, but the quantization noises of both modulation methods are the same in the high frequency. The simulated results show that the SNR of the SDM is 20 dB higher than that of the PCM, though the SDM modulation system is more complex than the PCM.