The glass transition temperature (Tg) of individual electrospun polymer polyvinyl alcohol fibers of varying diameter was measured using atomic force microscopy (AFM) based nanomechanical thermal analysis. Indentation and bending of individual electrospun fibers using AFM allowed the calculation of the elastic modulus of the polyvinyl alcohol (PVA) fibers across a range of different temperatures. The elastic modulus of electrospun PVA fibers was observed to decrease significantly when passing through Tg, which allowed accurate determination of Tg. The Tg of electrospun PVA fibers was shown to decrease for smaller fiber diameters especially for fiber diameters below 250 nm. This size-dependent glass transition behavior of electrospun PVA fibers is indicated as being due to polymer chain confinement. (C) 2011 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys, 2011