The kinetics of CuBr-mediated homogeneous atom transfer radical polymerization (ATRP) of methyl methacrylate (MMA) using 2-hydroxyethyl-2-bromoisobutyrate (HEBIB) as initiator and N-(nhexyl) pyridylmethanimine (NHPMI) as ligand was investigated. The experimental results showed that initially added Cu(II) can have strong effects on the kinetics of the ATRP depending on the [Cu(II)]0/[Cu(I)]0 ratio. When e10% Cu(II) relative to Cu(I) was added at the beginning of the polymerization, the kinetics can be well described by Fischer's equation (ln([M]0/[M]) µ t2/3).9 The obtained reaction orders for initiator, Cu(I) and Cu(II), are quite close to or the same as those in Fischer's equation, verifying the applicability of Fischer's equation in ATRP systems. On the other hand, when [Cu(II)]0/[Cu(I)]0 g 0.1, the kinetics can be well interpreted by Matyjaszewski's equation (ln([M]0/[M]) µ t).8 The polymerization rate shows almost first order with respect to the concentrations of initiator and Cu(I) and inverse first order with respect to the concentration of Cu(II), suggesting that the "self-regulation" and radical termination becomes unimportant for the ATRP process when enough Cu(II) is added at the beginning of the reaction. This result is of great potential importance for better control of ATRP systems. Besides, the equilibrium constant Keq and termination constant kt of the ATRP system at 90°C were determined to be 7.2 10-8 and 8.9 107 M-1 s-1, respectively.