Polystyrene clusters were prepared by using a trace amount of 4-methacryloyloxy-2,2,6,6-tetramethylpiperidine-1-oxy (MTEMPO) as a branching agent. Such clusters can undergo a thermal decomposition into linear chains at temperatures higher than 100℃. The thermal decomposition was studied by a combination of static and dynamic laser light scattering (LLS). The time dependence of the weight-average molar mass (Mw), the root-mean-square z-average radius of gyration (〈Rg2〉z1/2), and the average hydrodynamic radius (〈Rh〉) was used to monitor the decomposition kinetics and cluster structure. It has been found that Mw∝t-R, and the decomposition can be roughly divided into three stages; namely, from large clusters to smaller ones; from smaller clusters to less-branched ones; and finally to short linear chains. The scaling of 〈Rg〉∝Mw 0.33 (0.01 in the first stage indicates that these clusters are uniform in density, which is rare and much different from conventional polymer clusters whose density decreases from center to periphery. Moreover, we observed, for the first time, that 〈Rg〉/〈Rh〉∝Mw-0.20 ( 0.01, revealing that even for a uniform cluster swollen in a good solvent, its periphery is still more hydrodynamically draining.