A new tensile tester using electrostatic force grip was developed to evaluate the tensile strength and reliability of thin film materials. The tester was constructed in a SEM chamber for in-situ observation, and was applied to the tensile testing of polycrystalline silicon (poly-Si) thin films with dimensions of 30-300mm in length, 2-5 mm in width and 2 mm in thickness. It was found that the mean tensile strength was 2.0-2.8 GPa depending on the length of the specimens, irrespective of the specimen width. These size effects on the tensile strength showed that the location of the fracture origin was on the edge of the specimen, which was identified by the SEM observation of the fracture surface of the thin films.