Optical resonator biosensors have emerged as one of the most sensitive and practical microsystem biodetection technology. Here, we have developed a model for an optical microring resonator to be used as an ultrasensitive biosensor. A linear correlation between increasing the radius of the microring and the red shift in the resonance wavelengths has been observed. In fact, resonance shifts for very small changes in microring radius, as low as 10 nm, have been detected. Furthermore, sensing capability of the resonator has been simulated by introducing TiDO2 nanoparticles and protein molecules to the resonator surface by varying both thickness and effective refractive index of the attached layer such that the layer size has been changed from 10 nm to 100 nm with an increment of 10 nm. We have observed readily detectable unique resonance shifts for both TiDO2 nanoparticles and protein molecules. Moreover, effective medium approach has been implemented in order to account for refractive index fluctuations in sensing medium. As a consequence, combination of optical resonators with microfluidics could produce a simple-tooperate, portable and robust diagnostic tool enabling new insights into biomolecular function and recognition.