Abstract Ferroelectric potassium tantalate niobate thin films with composition KTa 0.65Nb 0.35O 3 (KTN) were grown by pulsed laser deposition on ( 1 1 ¯ 0 2 ) α-Al 2O 3 (R-plane sapphire) substrate. The effect of a KNbO 3 (KN) seed layer on KTN thin film growth was investigated. The crystalline quality, ( h 0 0) preferred orientation and in-plane epitaxial ordering of KTN thin films were greatly improved through the use of KN seed layers. KTN thin films with 14 nm thick KN seed layer showed optimized crystallization (Δ ω = 1.2°) and strong a-axis orientation. The microstructure investigation exhibited dense KTN thin films with a grain size of about 100 nm. The refractive index measured by ellipsometry at 632.8 nm was n 0 = 2.3 which is close to the bulk value of 2.34. Additionally, the presence of KN seed layer with nominal thickness (∼5 nm) fully suppressed pyrochlore phase which competes with KTN perovskite growth. Secondary neutral mass spectroscopy analysis indicated that interfacial diffusion depth is decreased by the use of this seed layer. All of these results evidenced the strong improvement of KTN thin film grown on sapphire by using a KN seed layer in view of microwave application.