The modal characteristics of dual-core photonic crystal fibers(PCFs) are analyzed by a full vector supercell lattice overlapping model. The fundamental and second-order modes of dual-core PCFs consist of a pair of even and odd modes with different polarization. The parity properties of the modal electric field for the fundamental and second-order modes are discussed. Based on the analysis of parity of the modal electric fields, we investigated the vector modal interference in dual-core PCFs.It is the interference between two same polarized modes that conributes to the inter-core coupling of power. It is shown that the optical power will oscillate from one core to the other and the phase difference between both cores is about π/2. The dependence of coupling coefficients for different polarization on wavelength is discussed.