Probe indentation techniques have been widely used to characterize the stiffness-related properties of polymeric materials over the past two decades. However, it still remains challenging to apply these methods for directly measuring the elastic modulus of nanometer-sized polymer films supported by stiff substrates, primarily due to the convolution between the polymer and substrate moduli. Here, we report on the influence of the probe size on the ability of these methods to directly quantify the elastic modulus of nanometer-sized polymer films supported by a stiff substrate. Particularly, we employ the nanomechanical mapping by atomic force microscopy (AFM) in combination with Johnson-Kendall-Roberts two-point method to measure the elastic modulus of Si-supported ultrathin poly(vinyl acetate) (PVAc) films of thickness ranging from 9 nm to 610 nm. For sharp AFM probes of similar to 12 nm in radius, this combination allows us to directly measure the elastic modulus of the polymer films as thin as similar to 18 nm without being affected by the stiff substrate. In contrast, larger probes of similar to 150 nm in radius show an obvious substrate-induced enhancement in the modulus of PVAc films with thickness similar to 100 nm, even when the indentation depth of the probe into the films is kept less than 2-3 nm. (C) 2016 Elsevier Ltd. All rights reserved.