We investigated the thermal evolution of the lattice parameters of a MnAs film epitaxially grown on GaAs(001) around its magnetostructural first-order phase transition using x-ray diffraction. Despite the substrate constraint, large variation of one of the in-plane lattice parameters is preserved, typical of bulk MnAs phase transition, during a large temperature range where two phases coexist. We demonstrated that the condition of the constant film length along this direction, in accord to the substrate length, is always fulfilled during the process. The effect is attributed to the gliding of misfit dislocations present on the film.8815