Although microwave integrated circuit(MIC) technology is employed for passive circuits, a tank circuit in the microwave band is still realized by a conventional cavity. It is our interest to consider a possibility of using a microstrip resonator as a low "Q" resonator in the microwave band. For this, the frequency shifting of resonators is studied. The resonant frequencies of resonators are influenced by various factors. In microstrip resonators, the factors contributing to the resonant frequency shift are fringe effect at open edges, dielectric loss, finite conductivity of metal plates, radiation loss from open edges, and loading at the input and output ports. The resonant frequencies of microstrip resonators are calculated using quasi-static approximation. In this approximation, a simple resonator model employing magnetic side wall is used. The theoretical calculation based on a quasistatic approximation are compared with the measurements performed at 2 to 4 GHZ range. This comparisons shows that calculations accounting fringe effect and loading effect fit well with the measurements. THe values of "Q" (unloaded) of the microstrip resonators go up to about 250, when Teflon fiber glass substrate is used at 3 GHZ range.