The mechanical bulge ductility tester developed previously by the authors (Nakahara et al.) is currently being considered by ASTM as a potential candidate for the standard ductility test apparatus for thin metal films. This paper describes (1) modifications and improvements made recently to the tester, (2) a theoretical analysis of the dependence of measured ductility on the size of the ball used to deform the specimen, and (3) the application of this tester to a study of the relationship between ductility and microstructure for copper films deposited by electroless and electrolytic methods.