The density functional theory method has been used to study the mechanism for the intramolecular hydrogen transfer (IHT) reaction of 1-methylbutly peroxide radical [n-C3H7CH(CH3)OO center dot]. The B3LYP method was used in conjunction with 6-31+G**, 6-311++G** and aug-cc-pVDZ basis sets. The geometrical configurations of reactants, productions and transition states were fully optimized on the potential energy surfaces. The results suggest that the IHT reactions involve the formation of transition states with five-, six- and seven-member rings when the transferring hydrogen atoms are at different initial positions related to the peroxide oxygen. The activation energies for the IHT reactions of beta-, gamma- and alpha-H migrations are 90.0, 100.8, and 140-152 kJ/mol at the B3LYP/6-311++G** level, respectively. The alpha-H is easier to transfer than the alpha- and gamma-H. Both the activation energy and rate constant for beta-H migrations at the B3LYP/6-31+G** level are basically in agreement with the experimental values. The methyl substituent on beta-position slightly lowers the activation energy, which is consistent with the experimental fact. All the IHT reactions are endothermic. The beta-H transferring is the most favorable process in view of both thermodynamics and kinetics. The predicted rate constants indicate that only the beta-H and gamma-H transfer reactions occur when the temperature is not high enough.