The magnetic force is essentially in proportion to the mass density and the magnetic susceptibility of paramagnetic chemical species and the gradient of magnetic field. In this study, the effect of magnetic field on OH radical distribution in a hydrogen-oxygen diffusion flame was investigated by numerically solving the equations of reactive gasdynamics and magnetism to explore a possibility of combustion control by magnetic force. Particularly, we attempted the analysis for three kinds of cases with different spatial distributions of the magnetic field: Case 1 is without the magnetic field, Case 2 is under the vertical gradient of magnetic field, and Case 3 is under the horizontal gradient of magnetic field. According to the analysis, because the mass density and the magnetic susceptibility of O-2 are much larger than those of other chemical species in the peripheral region of the flame, O-2 was strongly influenced by the magnetic force in that region. Consequently, this force induced the flow of gas including a large amount of O-2 and indirectly changed the OH distribution in the flame. The horizontal magnetic gradient (Case 3) was about 1.2 times more effective in the OH density change than the vertical gradient (Case 2).