Lead-free (K,Na)NbO3 (KNN) piezoelectric films were fabricated on Nb:SrTiO3 substrates by sol-gel process, and the influence of thermal processing details on the microstructure and ferroelectric property was investigated. Experimental results confirmed the different appearance of polycrystalline growth and epitaxial growth modes during the crystallization of KNN thin films. These two growth modes occurred under different thermal conditions within the narrow pyrolysis temperature scope ranging from 400 degrees C to 500 degrees C, respectively. By considering these findings, an optimizing combination between pyrolysis and annealing procedure was thought to be crucially important to obtain high-performance KNN films with excellent crystallization state.