The mechanical properties of ultrathin electrospun poly(L-lactic acid) (PLLA) fibers were studied by performing tensile tests on individual fibers. The tests were carried out on microelectronic mechanical systems (MEMSs) which were developed for characterizing the mechanical properties of thin polymer fibers. Force-displacement curves were obtained from video recordings of the experiments which were carried out in a scanning electron microscope. Each video was processed with an image processing routine to determine the elongation of the fiber and the elastic deformation of a component of the MEMS which yielded the force acting on the specimen. PLLA fibers with diameters ranging from 150 nm to 2 mu m were tested, The elastic modulus and the ultimate tensile strength of the fibers increased significantly for fiber diameters below 1 pm. This indicates a higher degree of orientation of the polymer chains in ultrathin fibers. (C) 2009 Wiley Periodicals, Inc. J Appl Polym Sci 114: 3774-3779, 2009