Photocatalytic degradation of trifluralin (C13H16F3N3O4) was examined in an aqueous suspension of TiO2 by using a Pyrex annular slurry photoreactor. Effects of TiO2 content, pH and O-2 flow rate were investigated. Experimental results show that the O-2 flow rate has no effect on trifluralin degradation at a pH value of 3, but when the initial pH of the suspension was 11, the percent removal of trifluralin was decreased by increasing the O-2 flow rate. It was also observed that the initial pH of the suspension had a significant effect on the photocatalytic degradation. The optimum TiO2 content, pH and O-2 flow rate for the most efficient degradation of trifluralin were found by the Box-Wilson optimization method as 2.7 g/l, 3 and 1.9 l/min, respectively.