The isochronal mechanical relaxation spectra of lightly cross-linked natural rubbers were measured at frequency 1 Hz. The results were analysed using the empirical Havriliak-Negami (HN) equation. The HN equation fitted the relaxation spectra very well. The fitting parameters, alpha and beta, obtained from the data fitting were used to interpret the relation between molecular motions and structure of polymers based on the model proposed by Schonhals and Schlosser. In the high-frequency region of relaxation spectra, the values of alpha beta were increased with increasing sulphur content. According to the model proposed by Schonhals and Schlosser, it means that the molecular motions are related to the local intramolecular interactions and are mainly influenced by the formation of heterocyclic groups along the chain. However, in the low-frequency region of relaxation spectra, the values of alpha were not affected by increasing sulphur content. According to the model proposed by Schonhals and Schlosser, the molecular motions in this region are related to the intermolecular interactions. They are not influenced by the level of cross-linking due to the very low level of cross-link.