Magnetron sputtered Ni thin films on both oxidised Si (100) and alpha-Al2O3 (0001) substrates of thickness 150-1000 nm were tested thermomechanically with a wafer curvature system, as well as in situ in a transmission electron microscope. The films on oxidised Si have a {111}-textured columnar microstructure with a mean grain size similar to the film thickness. On (0001) alpha-Al2O3 a near single crystal epitaxy with two growth variants is achieved leading to a significantly larger grain size. The thermo-mechanical testing was analysed in terms of the room temperature/high temperature flow stresses in the films and the observed thermoelastic slopes. It was found that the room temperature flow stresses increased with decreasing film thickness until a plateau of similar to 1100 MPa was reached for films thinner than 400 nm. This plateau is attributed to the present experiments exerting insufficient thermal strain to induce yielding in these thinner films. At 500 degrees C the compressive flow stresses of the films show a competition between dislocation and diffusion mediated plasticity. A size effect is also observed in the thermoelastic slopes of the films, with thinner films coming closer to the slope predicted by mismatch in thermal expansion coefficients. It is put forward here that this is due to a highly inhomogeneous stress distribution in the films arising from the grain size distribution.