Many theoretical studies have been proposed to understand the mechanism of pentaerythritol tetranitrate (PETN) coarsening. Up to date, no experimental observations of diffusion have been made. We present experimental evidence of diffusion of PETN at nanoscale, as observed by UV-absorbance spectroscopy. As a result of thermal gradient, non-isothermal heating of continuous PETN nanofilms results in an initial increased absorbance at ambient temperatures indicating thickness increase due to coarsening, followed by absorbance decease due to film sublimation at relatively higher temperatures. Diffusion kinetics of PETN is measured both isothermally and non-isothermally and the results are in very good agreement.