Internal dynamics of polymer chains is one of the most important fundamental problems in polymer physics. This work reports the first example of a dynamic light scattering (DLS) study of internal motions of well-defined ultralong comb-like chains in dilute solutions under good solvent conditions. Using two well-characterized comb-like model samples with controlled parameters and narrow distributions (M-w/M-n approximate to 1.30 and M-w approximate to 10(7) g/mol), the asymptotic behavior of the reduced first cumulant [Gamma* = F(q)/(q(3)k(B)T/eta(0))], the number of relaxation modes in Gamma(q) distribution curves, the characteristic relaxation time (tau(1)) of the first internal mode, and the q-dependent relative strength of internal motions have been experimentally quantified and carefully analyzed where q is the scattering vector. Specifically, the DLS results reveal the following: (i) Gamma*(q >> 1), that is, the value of Gamma* at the very high q regime, is not sensitive to structural details (local rigidity and side chain length); (ii) similar to hyperbranched polymers, the asymptotic power law in the regime of 3.0 < (q < R-g >) < 5.5 is also found to be a better indicator, which could reflect the structural details for comb-like polymers; (iii) the comparison of experimentally determined and theoretically calculated values of tau(1) shows that the classical theory of linear chains could still satisfactorily describe the intrachain relaxation behavior of comb-like chains; and (iv) the side-chain grafting-induced rigidity could result in an enhanced separation of translational and internal motions but simultaneously result in a significant suppression of the strength of overall internal motions. The present study not only reveals that the modes of internal motions of a given system are mainly determined by the relaxation of the main contour of a given chain structure rather than the local chain rigidity but also provides useful experimental data for further theoretical modeling of internal dynamics of various topological polymers.