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We have investigated the pressure-induced structural phase transition of orthorhombic and monoclinic KNbO3 nanowires by using x-ray diffraction (XRD) and Raman spectroscopy. Monoclinic KNbO3 nanowires underwent phase transitions three times: monoclinic to tetragonal, tetragonal to cubic, and cubic to orthorhombic-2 phase near 2.2 GPa, 8.2 GPa, and 40.0 GPa, respectively. Meanwhile, orthorhombic KNbO3 nanowires showed a phase transition once, orthorhombic to cubic phase near 8.5 GPa. Their exact structures were determined by the Rietveld method and confirmed by Raman spectroscopy. P-V equations of state for two kinds of KNbO3 nanowires were also derived from XRD data. Moreover, the elastic properties of the two nanowires in various structural phases were calculated. It was observed that two polymorphic nanowires, one from the monoclinic phase and the other from the orthorhombic phase, might have different isothermal bulk moduli even in the same crystal phase under high pressure.