Detailed ab initio density-functional calculations were carried out on the effect of lattice and internal relaxation of ZnO thin film under in-plane strain. It was found that the classical elastic deformation behaves under a restricted joint mechanism including lattice relaxation and internal relaxation. The internal relaxation works to enhance the effect of the in-plane strain on the perpendicular direction and, reduce the residual stress and the strain energy and, thus finally elevate the energetic stability of the resulting structure. By contrast, the free lattice and internal relaxation could consume part of the elastic strain energy to lead to a more stable structure with a degenerated space-group symmetry and no residual stress in any direction. In addition, the free relaxation brings about some different behaviors in many respects relative to the elastic deformation, such as the smaller Poisson ratio, the decreased piezoelectric effect, a "Lambda" shaped variety in bandgap at the F point and the reduced shift in homogeneity of charge distribution between the two kinds of Zn-O bonds. (C) 2010 Elsevier B.V. All rights reserved.