The remediation of water contaminated with herbicide oxadiazon was investigated by applying modified Fenton reagent ($Fe^{3+}/H_2O_2$) at unadjusted pH. Experimental conditions in various reactions allowed for the disappearance of oxadiazon. When using Fenton reagent, the maximum degradation rate was obtained under exposure to UV irradiation as compared to dark condition in which the concentrations of ferric ion and hydrogen peroxide were 3 and 100 mM, respectively. After 5 h of photocatalytic treatment with Fenton reagent, 98.4% of the oxadiazon was degraded. The high degradation rate of this herbicide was achieved by hydroxyl radicals ($OH^{cdot}$), which were generated by the photocatalytic treatment. UV irradiation accelerated the degradation of oxadiazon in the water system in the presence of Fenton reagent. In addition, the disappearance of oxadiazon was enhanced due to the increased concentrations of ferric ion and/or hydrogen peroxide. For the photo-Fenton reaction, one metabolite (m/z 310) was able to be identified using the GC-MS technique due to dechlorination of oxadiazon. Conclusively, a remediation strategy using Fenton reagent could be applied to degrade oxadiazon from contaminated water.