Fourier transform infrared Spectroscopy (FTIR) was used to study healthy and diseased leaves of rice, corn, and broad bean. The results show that the infrared spectra of the leaves are mainly composed of the absorption bands of proteins and polysaccharide. The spectra are similar, but the differences were observed in the peak positions, shapes, and absorption ratios. The remarkable differences in correlation coefficients were also observed in the region of 1 800~1 000 cm-1 of the second derivate spectra. The absorption ratio A1 056/A1 652 of healthy rice leaves and rice blast leaves, A1 652/A2 920 of healthy broad bean leaves and leaves, A1 056/A2 920 of healthy corn and corn rust (non-lesion site), and A1 652/A2 920 of healthy corn and corn rust (lesion site) all show a decline from healthy to diseased leaves, showing that the contents of polysaccharide and the protein have some differences.