The kinetic chemistry of H-abstraction reactions of isobutene by the OH radical and the channels of the C4H9O complex formed by addition of the hydroxyl to isobutene were investigated in current work. Their rate constants at high pressure limits were computed based on the canonical transition state theory with tunneling effect considered, following the construction of the potential energy surface (PES) based on a duel-level quantum chemistry method at CCSD(T)/CBS//BHandHLYP/6-311G(d,p) level. Comparison of reaction rate constants of H-abstractions by the OH radical for isobutene with previous experimental and calculated data showed acceptable agreements. Isomerization pathways of the hydroxybutyl radicals to each other and to the isobutoxy radical were discussed in details. It was observed that the rate constants for both H-abstraction and isomerization via Htransfer were improved by considering tunneling effect. Subsequently, the pressure-dependent rate constants of multiple channels for each hydroxybutyl radical and of the dominant channel for isobutoxy radical were calculated based on RRKM/ME theory. The effect of chemical activation and thermal initial distribution was discussed with the case of 2-hydroxymethyl-prop-2-yl (W1) radical.