A compact tooth-shaped grating-assisted structure has been proposed for a multimode interference (MMI) coupler and studied theoretically with silica waveguides with silicon oxynitride cores by using a mathematical model based on the sinusoidal mode simple effective index method. The dependence of the access waveguide length and beat length of the grating-assisted MMI (GA-MMI) structure on access waveguide separation (h) is shown. The total length of the GA-MMI coupler is less than that of a grating-assisted two-mode interference (GA-TMI) coupler (h=0) for an access waveguide S bending loss of 0.2 dB. It is also seen that the effect of fabrication tolerance on power imbalance of a GA-MMI coupler is almost close to that of a GA-TMI coupler.