The photocatalytic decolourization of C.I. Basic Red (BR46) was carried out in a thin film fixed bed photoreactor with external UV light irradiation. The optimization of UV/TiO2 process was carried out using full factorial experimental design (24) approach in order to assess individual and interactive effects of the four main independent parameters (initial dye concentration (mg/L), Intensity I (w.m-2), flow rate (L.min-1) and reaction time (min) on the decolourization efficiency Y (%), Experimental findings were in close agreement with the prediction model (R = 99.57 %, Adj-R² = 0.987) and the photocatalytic degradation efficiency Y (%). The effects of individual variables and their interaction were determined using ANOVA analysis. The results indicate that the reaction time and the light Intensity exhibit a significant positive effect on the efficiency of decolourization, whereas the initial dye concentration and the flow rate show a significant negative effect. Moreover, the interaction between light Intensity and reaction time was the most influence one, the experimental values agreed with the predicted ones, indicating suitability of the model employed. The maximal decolourization efficiency of 99.95 % was achieved at initial dye concentration 20 mg/L, UV light intensity 38.1 W/m2, flow rate 0.3 L/min and reaction time 150 min. Thus, under this condition the highest TOC removal efficiency of 61 % mineralization was obtained.