The combustion characteristics of laminar biogas premixed and diffusion flames in the presence of upward decreasing magnetic fields have been investigated in this study. The mechanism of magnet−flame interaction in the literature, in which magnetic fields change the behaviors of laminar flames due to the paramagnetic and diamagnetic properties of the constituent gases, is examined and the results are as follows. The magnetic field has no noticeable effect on premixed flames due to low oxygen concentration of the mixed gas at the injection and the relatively high flow momentum. However, due to the diffusion nature of diffusion flames and paramagnetic property of oxygen in ambient air, oxygen distributions are subjected to the gradient of magnetic flux, thus shortening the height of diffusion flames. Results also show that the flame volume is more strongly varied than flame height. Altered oxygen distributions result in improved combustion and higher flame temperature. In the case of current magnet−flame interaction, the magnetic driving force is combined with gravitational force, and a modified gravity g* as well as gravity modification factor G are derived to characterize the paramagnetism theory of oxygen.