Attenuated total reflection (ATR) FTIR spectra of benign and malignant breast tumor tissues from 32 female patients have been determined and investigated. The spectral differences and changes (shape, peak position and relative intensities) have been observed between these two types of samples. In comparison with the benign tumor tissues, the mainly characteristic changes of malignant breast tissues involve: 1) The secondary structure of proteins in malignant tumor tissues is composed mainly of α-helix and random coil, in which α-helix structure is dominant. 2) The absorption band near 1160 cm~-1 due to the C-O(H) stretching modes of serine, threonine, tyrosine groups evidently shifted to a higher wavenumber, suggesting that the intermolecular hydrogen bonds were nearly broken, and the collagen absorption peaks at 1205 cm~-1 , 1278 cm~-1 and 1338 cm~-1 became lower or disappeared, showing its relative contents decreased. 3) The bands from the symmetric and asymmetric stretching modes of the PO-_2 of the phosph