The aim of this work was to improve photo-catalytic activities of zinc oxide thin films for the degradation of methyl orange (MO) using a new approach based on the combined effect of lanthanum doping and porous substrate of silicon (Psi). Therefore, undoped and lanthanum-doped zinc oxide thin films (ZnO : La) were deposited on porous silicon substrate (PSi) by spin coating chemical deposition. Studies of the photo-catalytic activity showed that the La (25 %)-doped ZnO layer exhibited an improved photocatalytic performance in the degradation of methyl orange. Around 91.53 % and 70 % of the MO degradation was achieved within 180 min under illumination with UV and visible light respectively, suggesting that La-doped ZnO films deposited on Psi substrate are favorable candidates to enhance the photo-degradation of methyl orange.