Abstract: A two-dimensional pseudo-homogeneous model of reactor was set up for the study of industrial reactor for making methacrylic acid by one step oxidation of isobutanal. To solve the model, orthogonal collocation method was used. The axial and the radial distributions of the temperature and the yield in the bed were obtained by using Gear method. The results of simulating the industrial single-tube reactor agree well with those from the measurements. By simulation,the optimized operational condition and the suitable operatable range were determined for the isobutanal reactor of 6 000 tons/year productivity.