The studies of optical nonlinearities of 2-(2′-hydroxyphenyl)benzoxazole(HBO), based on excited state intramolecular proton transfer in cyclohexane solution using Z-scan technique are reported. The results show that: (1) At 1.06 μm, HBO has no nonlinear absorption. And its real part of third order nonlinear susceptibility, χ(3)R is not dependent on the pumping intensity I0; (2) with the irradiation at 0.53 μm, HBO exhibits evident two photon absorption (TPA). Its TPA coefficient decreases with increasing I0, while its χ(3)R increases with increasing pumping intensity I0. By establishing the ESIPT kinetic model of HBO based on TPA, we can explain the experiment satisfactorily.