We present a method to measure the in-plane thermal conductivity of thin films by the laser flash technique. The method uses a well-defined structure for the analysis. We have realized the structure by conformal deposition of ZnO films of different thicknesses using atomic layer deposition onto a 20 μm thick ion track etched polycarbonate membrane as substrate. By using this procedure we could determine the thermal conductivity of the deposited thin film from the total thermal diffusivity of the nanocomposite structures. The method has been used to obtain the in-plane thermal conductivity of the deposited ZnO layers within the thickness range of less than 100 nm.