Microring resonators are strong candidate for the basic building blocks of very large scale integrated optics. They can be used in many applications, such as filters, routers, switches, lasers and amplifiers. They are simple in design and concepts, can be made very small and do not require exotic materials on fabrication techniques. InP was chosen to as the base material for the ring since it belongs to IIIV semiconductor which are perfect in this field since they have high refractive indices (>3), small bending radii, fast response time, and the are optically active. In this thesis, a mathematical model was driven for the microring resonator starting from its basic element (directional coupler ). Then the effect of varying the coupler parameters on the spectral response, hence the output powers of the ring was shown. Finally the main characteristics ( Resonance Width, Free Spectral Range, Finesse, the Quality Factor and Intensity Enhancement ) of the ring was driven and how the variation of the coupler parameters will vary such characteristics was shown. The simulation results are carried out using MATLAB 7 package.