We propose a new method to measure magnetostriction of magnetic thin films using piezoelectric ceramic substrates and some feedback circuits. Conventional methods are based on the measurement of the bending of a cantilevered substrate sample with a magnetic film when a magnetic field is applied in the plane of the film. The main idea of the developed method is to “compensate” any bending along the sample's length, changing the length of the substrate using the inverse piezoelectric effect, by the same amount of the thin film's change of length (caused by the magnetostriction). A feedback loop keeps always the sample flat along the sample's length. This method, being a null-measurement method, depends on very few parameters, and does not depend on any physical constant of the thin film being measured. It measures the magnetostriction directly and permits, in principle, far greater accuracy and reliability than the conventional methods. Magnetostriction of Ni thin films were measured, and the values obtained are very close to the saturation magnetostriction of bulk Ni crystals.