To study the characteristics of breakdowns in different materials under a high RF field, a high-gradient experiment was proposed with using narrow waveguides having a field of around 140 MV/m at 50 MW power. Two experiments, one on copper (OFC) and another on stainless-steel (AISI-316L), were done. The breakdown rate of the copper waveguide was found to be larger than that of the stainless-steel waveguide at fields higher than several tens of megavolts per meter. The surfaces of both materials were observed after high-field experiments. The area suffering from heavy breakdowns was found to be rugged for the copper waveguide, while that of the stainless-steel smoother than the copper case, even though the heights of dips and bumps were almost the same at 20∼30 microns for the two materials. From these results, it was proven that the breakdown study using narrow waveguide under the high gradient field offered great potential. © 2010 The Institute of Electrical Engineers of Japan.