Atomic force microscopy has been applied for measuring the nanomechanical characteristics of poly (methyl methacrylate) thin films containing 5% photoinitiator (Irgacure 651). The nanohardness, Young's modulus, and adhesion to AFM tip have been evaluated for the unexposed samples and after UV-irradiation. Additionally, FTIR spectroscopy and differential scanning calorimetry (DSC) have been applied to explain the observed changes in nanomechanical properties. It was found that the exposure to ultraviolet changed the nanomechanical properties of polymer because of photo-oxidative degradation and relaxation processes. These studies lead to the conclusion that the applied photoinitiator has no noticeable effect on nanohardness and Young modulus during PMMA irradiation, but efficiently participates in polymer photo-oxidation increasing the surface hydrophilicity and adhesion to Si(3)N(4). Moreover, the initiator hampers the relaxation of PMMA macromolecules, what was proved by DSC. (C) 2011 Wiley Periodicals, Inc. J Appl Polym Sci 123: 2458-2466, 2012