The transmission characteristics of the symmetrically confined photonic crystal (PC) defects had been detailedly investigated. By using the parameters of their defect modes, we can readily get the transmission characteristics of the asymmetrically confined photonic crystal defects. It shows that in the framework of coupled-mode theory, an asymmetrically confined photonic crystal defect can be considered as the combination of two different symmetrically confined photonic crystal defects, and its transmission characteristics are decided by the decay-rates of the two involved symmetrically confined defects. All the analyses are in a good agreement with the simulation based on the finite-difference time-domain technique.