Microring resonator has become a hotspot due to its functionality, compactness, and dense integration. The microring resonators provide passband characteristics of steeper roll-off, flatter top and greater stopband rejection than single ring resonators. Both the transfer functions of series-coupled and parallel-coupled multi-ring microring resonator filters are derived by the signal flow graph method. For the series-coupled structure, the filtering response of filter is determined by the coupling coefficients between rings and the optical loss. For the parallel-coupled structure, the good shape of spectral response is achieved by optimizing the ring distance.