A phase distortion method is applied to examine the optical nonlinearity in tellurium dioxide single crystal. The positive nonlinear index of refraction is observed in the crystal at above the two‐photon absorption edge. While the observed positive sign is consistent with that predicted theoretically, the absolute values of both nonlinear index of refraction and two‐photon absorption coefficient are larger than the theoretical values by a factor of five. ©1995 American Institute of Physics.