High quality and high quantity ultra-long KNbO3 microwires with diameters of 200 nm to a few microns and lengths up to several milimeters have been synthesized by using a simple solvothermal method. The high purity, ideal length and good transferability make it easy to be transferred to the quartz substrate for further linear and nonlinear optical measurements. Functionalized by plasmonic Au nanoparticles, the as-prepared KNbO(3) sub-microwires exhibited enhanced anisotropy of nonlinear absorption that the polarization ratio of individual Au nanoparticles coated KNbO(3) sub-microwire (ρ=0.32) is two times more than the uncoated one. It is demonstrated that this ultra-long KNbO(3) microwire is an ideal candidate for nanolasers, polarization sensitive photodetector and photo-thermal nanodevices.