The main aspect of this work was to gain further understanding of the mechanical properties of thin films using thermomechanical experiments. Al thin films with different thicknesses, grain sizes and textures were thermomechanically cycled and isothermal relaxation experiments during cool down were performed. During the thermomechanical cycling the microstructure is characterised. In the first part of this thesis stress-temperature curves from different Al thin film system are investigated to study their heating up and cooling down behaviour. Different film thickness up to 2μm of fine and coarse grained Al films on Si substrates and epitaxial Al films on α-Al2O3 substrates were investigated. As a result of their different microstructures a different behaviour of the recorded stress-temperature curves is observable. Coarse grained polycrystalline Al films shown a stress-temperature hysteresis typical for films without a passivation layer. In contrast, the fine grained and epitaxial Al films show the expected behaviour for films with a passivation layer. With the help of theoretical considerations the stress-temperature curves can be classified as dislocation- or diffusion-types. The second part concentrates on the thermomechanical fatigue experiments and the influence of the microstructure on the formation of damage phenomena. As a consequence of the amorphous interface with the polycrystalline Al films, the damage phenomena can be explained by dislocation interactions at the amorphous interface. Polycrystalline Al films also show a film thickness dependent behaviour. Al films with thicknesses ≥600nm exhibit severe deformation of their surfaces after 10000 thermal cycles. Al films with thicknesses below 600nm showed no reactions at their surfaces due to the thermal treatment. The initial texture plays an important role in the thermomechanical fatigue behaviour. While epitaxial films have no reactions due to the thermal treatment at surface or in texture, a different behaviour is observed with polycrystalline Al films. The polycrystalline Al films show a rotation of their initial (111) orientation towards a (112) orientation. The lattice rotation is not only observable after 10000 thermal cycles. It was possible to detect the lattice rotation of the polycrystalline Al film with a thickness of 600nm at single grains after several thermal cycles. The final section is concerned with the isothermal relaxation experiments during cool down at a stress-temperature cycle. It was possible to obtain activation energies and activation volumes of polycrystalline and epitaxial films. From the collected data the dominant relaxation mechanism up to 250°C is an obstacle controlled glide of dislocations. At 250°C there is a change in the deformation mechanism observed. The calculated activation energies for the relaxation mechanism above 250°C of about 0.3eV were found to be controlled by interface diffusion or a double kink mechanism.