Using electrospinning technique polystyrene (PS) nanofibers in the thickness range from 150 to 800 nm have been produced. Electron microscope inspections reveal the relatively uniform thickness of the obtained fibers. The mechanical deformation mechanisms have been studied in tension tests using micro-tensile devices for a scanning electron microscope (SEM) and a transmission electron microscope (TEM). A characteristic change in the deformation behavior from the typical craze formation of PS to a micro necking and cold drawing has been found with decreasing fiber thickness. There is a surprisingly sharp fiber thickness limit between both deformation types in the range of 220225 nm: nanofibers thicker than similar to 225 nm deform with formation of crazes, nanofibers thinner than similar to 225 nm show necking and cold drawing. (c) 2012 Wiley Periodicals, Inc. J Appl Polym Sci, 2012