In order to evaluate strength of micron size polycrystalline silicon (poly-Si) structure for MEMS considering surface morphology difference between top and sidewall and effective surface area, bending strength tests of cantilever beam, surface roughness measurement and fracture surface analysis are performed. The specimens are made by CVD process for poly-Si deposition and deep RIE process for sidewall formation, and then the surface morphology of the top and the sidewall surface are different. The various size notches on the specimen are introduced in order to change effective surface area. By the fracture surface analysis, it was found that the fracture initiation point was not always maximum stress point; this is because there exist stress concentration on the surface. Surface roughness was measured using atomic force microscope (AFM). Then the maximum stress concentration of the specimen on the top and the sidewall surface respectively were presumed using extreme statistics, and effective surface area was defined. Then, bending strength and effective surface area shows good correlation.