A novel magneto-optical (MO) waveguide with a nanoscale air gap (NAG) is presented. By using an NAG, the nonreciprocal phase shift (NPS) of the MO waveguide can be enhanced. A linear transition model is proposed to theoretically explain the NPS of the MO waveguide with the NAG. Simulation result shows that when the NAG thickness is less than 10 nm, the MO waveguide exhibits an NPS more than four times that of the MO waveguide without the NAG.