Considering that KBe2BOsF2 (KBBF) is an outstanding deep ultraviolet crystal which can generate the shortest wavelengths in second harmonic generation and sum-frequency generation, we report the determination of the nonlinear optical coefficients of the KBBF crystal. The d11 coefficient was determined to be 0.49pm/V by the Maker fringes method at the wavelength 1064nm, which is in agreement well with the theoretical value.