Abstract: The resonant photonic modes in a quantum well (QW) structure composed of three slabs of different photonic crystals with different lattice constants is analyzed with planewave based transfer matrix method (TMM). It is found that the energy band of the well slab submerged into the band gap of barrier slab is discretized into resonant modes and the number of the resonant modes changes with the well slab thickness. A model structure of asymmetrical photonic QW consisting of two slabs of 2D photonic crystals with different lattice constants and one uniform dielectric slab in between is firstly proposed and the resonant modes in it are investigated. A useful numerical simulation method for theoretical discussion as well as for practical application about photonic QW structure of photonic crystals with different lattice constants is proposed.