A new tunable OADM based on photonic crystal ring resonators (PCRRs) is proposed by changing the silicon rod index of ring region.The provided PCRR also satisfies the well-known relationship between free spectral range (FSR) and ring radius R of traditional micro-strip ring resonators.The effect of index change on the dropped wavelength and its corresponding efficiency are then numerically analyzed by using two-dimensional finite-difference time-domain (FDTD) technique with perfectly matched layers (PML) as absorbing boundaries.1-nm dropped wavelength shift can be obtained with only 0.005-Δn and less than 1.4 μm effective ring radius.