Abstract: This paper presents a photonic crystal add-drop filter (ADF) based on ring resonators. The filter was designed in a 2D square lattice dielectric cylinder photonic-crystal structure which consisted of one input port and two output ports. The transmission properties of the ADF with one photonic crystal ring resonator (PCRR) and two PCRRs were studied by time-domain finite-difference method. The results of investigations show that this structure has high drop efficiency in the third communication window, and the signals dropped by ADF can be propagated along upward, downward, forward and backward directions. This structure has the advantages of tunable switch wavelength, broad band, and high normalized transmission rate. It is applicable in the fields of photonic crystal integration and optical device manufacture.