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