Abstract: It is proposed that only the heteronuclear cross relaxation rates between ¹³C and ¹H,which are only dependent on the diporlar relaxation,are used to investigate the mo-lecular internal motions,The new method is more reasonable than the conventional one in theory where the nuclear longitudinal relaxation time T₁,transvcrse relaxation T₂ and NOE factor η are used to study the molecular internal motions. The general formu-la separating the contribution of dipolar relaxation to the total ¹³C spin relaxation is set up. It can be applied not only to small molecules inside extreme narrowing condi-tion but also to macromolecules outside extreme narrowing.

Key words: Macromolecule, Dipolar relaxation, Nondipolar relaxation, Nuclear cross relaxation, ¹³C spin relaxation