In this letter, a new compact ultra wideband circular monopole antenna with an acceptable band-rejection characteristic is investigated. This rejection band is created by means of an electromagnetic band-gap (EBG) structure. The EBG structure that is used here is a mushroom-like structure. An equivalent circuit model is employed to investigate the stopband characteristic of the EBG. The operation frequency band is 3.1-10.6 GHz with a rejection band of 0.7 GHz around 5.5 GHz. Accurate and high rejection is achieved. The proposed model is implemented, and the measured results are in good agreement with simulated ones.