Nonlinear optical properties of KNbO3 single crystals were investigated in detail in the orthorhombic phase by using YAG/Nd laser. The refractive indices for some laser wavelengths were measured. The nonlinear optical coefficients of this crystal at room temperature were obtained by means of the Maker fringes technique. Results were as follows: d31=35, d32=-40 and d33=-61 relative to d11 of quartz. These values are comparable with those of Ba2NaNb5O15. The temperature dependences of dij coefficients were measured. Critical and noncritical phase-matching conditions were determined. At room temperature, critical phase-matching was possible for d31 and d32 coefficients. The observed phasematching angles were found to be in good agreement with the values calculated from the refractive index data.