To understand the rf breakdown phenomenon better, an x-band coaxial dual-moded cavity is designed. It is independently excited two modes from two sources. One mode will generator pulsed heating in the inner conductor and the other one will co ncentrate peak electric field. By observing the breakdown rate and damage on the surface for different electric to magnetic field ratios, we hope to reproduce the limiting RF field effects seen in various accelerator structure, waveguides and klystrons. T he initial high power test has been done in SLAC. The experiment results will be discussed in the paper together with future experiments. Show Partial Abstract