The aim of the current study was to investigate the spectra in the different organs of the rats which died of massive hemorrhage; to explore their spectral changes 15 days postmortem and the best mathematical model with different band absorption ratio changes to postmortem interval(PMI); and to compare the spectral changes of different temperature. Thirty male Sprague-Dawley rats were sacrificed by cutting abdominal aorta, and the cadavers were divided equally and kept at 4 degrees C, 20 degrees C and 30 degrees C in the control chamber. From the same rat, seven different organs were sampled at intervals of 1-15 days postmortem, and then measured by Fourier transfom infrared (FTIR) spectrometer. Six mathematical model functions were explored. The absorbance of bands and band absorbance ratios of absorption peak in each organ showed a time-dependent increase or decrease, most band absorbance ratios remaining stable for 7-15 days postmortem. Cubic model functions of the various bands absorbance ratios against PMI showed a stronger related coefficient. The absorbance bands with obvious changes at 20 degrees C showed stabilized tendencies at 4 degrees C and significant changes at 30 degrees C within 15 days postmortem. In addition, FTIR spectroscopy revealed a time-dependent metabolic process, with potential of being used to estimate PMI during 7 days postmortem, which merits further investigation.