High-quality epitaxial KNbO3 (KNO) thin films grown on single crystalline (001) Rh substrates exhibited a tetragonally strained structure with a c/a ratio of 1.04. The strained KNO thin film showed a high remanent polarization of 42 mu C/cm(2) with a coercive electric field of 170 kV/cm. Based on the density functional theory calculations, the increase in polarization and piezoelectricity in the tetragonally and compressively strained KNO thin film is primarily attributed to the faster increase of the displacement of Nb ions than the decrease in the Born effective charge.