Polystyrene films, with thickness ranging from a few tens of nanometers up to several hundreds of nanometers and molecular weight of 27.5 kg mol-1, were patterned with nanoimprint lithography (NIL) technique. A rigid silicon stamp containing nanoscale features was printed into a thin spin coated polystyrene film. Then these patterns were annealed above the glass transition temperature in order to characterize the viscous reflow of the topography. Special attention was paid to provide, at initial times, imprinted nanoscale patterns with a very small aspect ratio and amplitude/wavelength as well as to avoid the nucleation of holes during imprinting or during the course of the reflow. This allowed the authors to process topography data with a high degree of accuracy from a linear viscous stability model. Atomic force microscopy measurements, with a spatial resolution lower than 1 nm, were used to characterize smooth or steep shapes. The mechanical measurements of earlier stages of pattern reflow were directly accessible without any assumption, contrary to the diffraction method usually employed. Our results clearly demonstrate that even the earliest stages of pattern reflow are driven by simple viscous effects and that relaxation dynamics, which is usually considered as following exponential laws, could be more complex. This article also demonstrates that the NIL process can be used for viscosity measurements for ultrathin resist film.