We propose a new theory of ultra-narrow bandwidth optical filter which consists of two-dimensional photonic crystal of an anomalous dispersion material. Based on our investigation of the anomalous dispersion of 6P1/2←6S1/2 tran sition of the cesium atom, we designed a photonic crystal anomalous dispersion o ptical filter (PCADOF) by use of the plane wave expansion method. The calculated result shows that the bandwidth of the filter can be less than 0.002nm.