The kinetic theories on the polymerization rate and polydispersity in atom transfer radical polymerization (ATRP) were, for the first time, quantitatively tested by experiments for the polymerization of styrene at 110 degreesC with a polystyryl bromide/Cu(I)Br/ligand initiating system in the presence or absence of the conventional radical initiator 2,2'-azobis(2,2,4-trimethylpentane) (VR110). The polymerization rate in the VR110-free system conformed to the theory on an absolute scale, showing a (2)/(3)-order time dependence of conversion index. The addition of VR110 increased the polymerization rate to reach a stationary state. Experimental results on polydispersity also quantitatively agreed with the theories in both the presence and absence of VR110, exhibiting the theoretically predicted time evolutions of polydispersity index characterized by the prefactors (1)/(2) and (3)/(8), respectively.