Polystyrene was degraded by using a vibrating ball mill. The viscosities and molecular weights of the degraded products were measured, and the decrease of viscosity $[eta}$ with ${dot{s}}$ (rate of shear) observed for the degraded products were analyzed by applying the Ree-Eyring equation for viscous flow. The variation of the parameters $x_2$/{alhpa}_2,{eta}_2$ and $x_1{eta}_1/{alpha}_1$ in the equation were explained by the fracture of polymer molecules by mechanical force. The electron paramagnetic resonance spectrum of the degraded sample was taken, and it was confirmed that free radicals were produced by the chain-scission of polystyrene.