The properties of two-fold images based on asymmetric interference in PC slab multimode waveguide with an air holes array are investigated. With these image properties, a new kind of 2×2 ultracompact power splitters (PSs) is designed and studied. Arbitrary splitting ratio can be achieved by modifying the effective refractive indices of one pair of air holes in the multimode region. As an example, a 3 dB ultracompact 2×2 power splitter with a size of 16 μm×8 μm, with output efficiency as high as 97% is presented with finite-difference time-domain simulations. The method can be extended for the design of M×N PSs and have potential application for photonic integrated circuits applications.