Nanocrystalline ZnO:Al thin films were deposited by reactive chemical pulverization spray pyrolysis technique on heated glass substrates at 450 degrees C to study their crystalline structure, composition, strain, stress, roughness characteristics and nonlinear optical susceptibility as a function of Al concentration (0, 2, 3, 5 at.%). The films were characterized by X-ray diffractometer (XRD), EDAX9100 analyser, atomic force microscopy (AFM) and third harmonic generation (THG). The Al (3 at.%) doped ZnO thin films exhibited the lower strain/stress than undoped films. The nonlinear properties of the ZnO: Al thin films have been found to be influenced by the films strain/stress. (C) 2011 Elsevier B.V. All rights reserved.