Obtaining high purity alkali niobate (KxNa1-xNbO3) thin films without secondary phase on metal coated traditional silicon (Si) substrates via sol–gel technique has remained great challenges until now. Herein, we report K0.5Na0.5NbO3 (KNN) thin films successfully deposited on Pt/Ti/SiO2/Si(100) substrates by a simply effective sol–gel process. A comprehensive and systematic investigation of processing conditions on the microstructures and electrical properties of spin-coated KNN films was presented. We have found that phase purity and microstructures of KNN films are strongly influenced by content of alkali excess and the annealing temperature. Thin films with an equal excess amount of 10% mol K and Na (KNN1) sintered at 650 °C show high crystallinity with a preferred (100)-orientation degree of 78%, and homogeneous and dense surface with columnar structure and large grain size up to 254 nm. The result of quantitative XPS analysis has proved that the composition of the film is close to the chemical stoichiometry. As a consequence, the obtained KNN1 films exhibit a large dielectric constant of 775 and low dielectric loss of ∼2% in the wide frequency range from 1kHz up to 10MHz as well as the best shape of P−E loops. Furthermore, leakage current density of the film is about 9.45 × 10−5 A cm−2 at E ≈100 kV cm−1.