A new tensile tester using electrostatic force grip was developed to evaluate the tensile strength and the reliability of thin film materials. The tester was constructed in a SEM chamber for in-situ observation, and was applied for tensile testing of polycrystalline silicon (poly-Si) thin films with dimensions of 30-300 mu m long, 2-5 mu m wide and 2 mu m thick. It was found that the mean tensile strength is 2.0-2.7 GPa depending on the length of the specimens, irrespective of the specimen width. Statistical analysis of these size effects on the tensile strength predicted that the location of the fracture origin is on the edge of the specimen, which is identified by the SEM observation of the fracture surface of the thin films.