The photodegradation of imazaquin (50 mg/L) was investigated under simulated sunlight experiments. This photodegradation was evaluated at pH=7 in the presence of humic acids (HA) isolated from Moroccan soil known for their interaction with organic and mineral components. The study was realized at different imazaquin/HA ratios (1/0.1; 1/0.5; 1/1). Irradiation was carried out with polychromatic light using cylindrical reactor equipped with HPK 125W lamp. The concentration of remaining herbicide was followed using a high pressure liquid chromatograph (HPLC) equipped with UV detector at 230 nm. The role of HA on the photodegradation of imazaquin was evaluated by the quantum yield determination. Kinetic parameters of photoreaction show a first order dependence of concentration of imazaquin versus time. The presence of HA induced a decrease of the photodegradation of the pesticide with respect to pure water solution. A short of protection and screen effect of humic substances on the pesticide molecule was observed.