We report a waveguide crossing based on a multimode-interference (MMI) structure for metal-insulator-metal (MIM) waveguides. The MMI-based crossing comprises two orthogonal intersecting MMI waveguides that are connected to the single-mode input/output waveguide symmetrically. Single self-images are formed at the crossing center and output plane of the MMI waveguide, thereby mitigating the crosstalk and improving the throughput. The characteristics of the proposed MMI-based crossing are investigated with the finite element method of Comsol Multiphysics. The results show that the throughput reaches $1.8\mathrm{dB}$ and the crosstalk is less than $46\mathrm{dB}$ at the wavelength of 1550 nm.