The glass transition temperature (Tg) and dynamic viscoelasticity of comb-like branched polystyrene (cPS) compared with linear polystyrene (lPS) were measured by DSC and rotational rheometer. The relaxation behavior of motion units for cPS was investigated. The results show that the Tg for cPS is higher than that of lPS with the same weight average molecular weight. When the cPS samples own equal backbone lengths, Tg increases as the side chain length or degree of branching is improved. Relaxation corresponding to backbone and side chain motion of cPS occurs in low and high frequency region, respectively, and the side chains delay the relaxation of the backbone. With side chain length increasing, the characteristic relaxation time of backbone and side chain becomes longer. With degree of branching increasing, the characteristic relaxation time of backbone turns longer and the characteristic relaxation time of side chain remains unchanged. The absence of the rubber plateau in these comb-like branched polystyrenes is ascribed to the side chains of high density around the backbone, which weaken the entanglement among the macromolecular chain. The entanglement among the side chains has little impact on rubber plateau. The existence of the side chains delays the relaxation process of the macromolecular chain of cPS, and leads to the storage modulus G’ of cPS at viscous flow state higher than that of lPS with the same weight average molecular weight.