Spontaneously appearing macroscopic polarization (selfpolarization) in ferroelectrics without an electrode or an external electric field would enable the freedom to design many ferroelectric heterostructures and devices. The (K0.5Na0.5)(Mn0.005Nb0.995) O-3 (KNMN) thin film was grown on Nb: SrTiO3 single-crystal substrate and the resultant selfpolarization behavior due to strain relaxation was investi-gated. The lattice mismatch and difference in TEC between the KNMN thin film and the Nb: SrTiO3 substrate creates a compressive strain. The compressive strain gradient may be the main cause for the observed downward self-polarization. The downward self-polarization of the KNMN thin film can be explained by the strong inhomogeneous compressive strain caused by strain relaxation.