Based on the comparison of the infrared spectra of polyamide homologues, on the changes in the infrared spectra of deuterated and iodine-treated samples of polycaprolactam, and also on the observations of the infrared dichroism of drawn film, the main absorption bands of polycaprolactam in the region 400 cm-1-1400cm-1, which are rather structure sensitive, have been assigned as follows: 1370 cm-1 band-νC-N; 1265 cm-1 and 1205 cm-1 bands-δN-H+ν(C-N)(Amide Ⅲ); 1170 cm-1 band-νC-N; 730 cm-1 band-γCH2 of (CH2)n when n≥4; 690 cm-1 band-δN-H (out of plane, Amide V); 580 cm-1 band -δN-H (out of plane).