Porous nanocrystalline ZnO photocatalysts were successfully synthesized by microwave irradiation and then thermally pretreated at different temperatures (150 oC, 200 oC, 250 oC and 300 oC). The physico-chemical properties of prepared samples were determined by thermogravimetric analysis, X-ray diffraction, scanning electron microscopy, transmission electron microscopy, N2-adsorption, FTIR spectroscopy and UV-vis absorption spectroscopy techniques. The characterization results indicated that the as-synthesized samples comprised of both ZnO and Zn(OH)2 agglomerates and had a particle size of approximately 50 nm. Thermal treatment of the as-synthesized sample at 150 oC resulted in a pure ZnO phase with particle size of 40 nm. The results also demonstrated that the surface area, pore diameter and bandgap energy reaches a maximum value for the sample treated at 200 oC. The photocatalytic activity of all the samples was evaluated by photocatalytic degradation of aqueous methyl orange solution at ambient temperature. The ZnO sample pretreated at 200 oC showed the highest photocatalytic activity in short reaction time (99%, 90 min.), which can be attributed to the combined effects of several factors including low crystallite size, relatively high surface area, pore diameter, pore volume and bandgap energy. Reusability results show that the catalysts can be readily separated from the reaction mixture by filtration after the photocatalytic reaction and reused at least five times without any loss of activity.