Passive radiative cooling is a promoting way for thermal energy management. High reflectance of solar light and high emittance in the atmospheric window (8-13 mu m) are acquired for effective radiative cooling. In this work, we demonstrate a daytime passive radiative cooler using chemically fabricated porous anodic aluminum oxide (AAO) membranes. Effective medium theory (EMT) has been applied to analyzing the optical properties of the air-doped porous medium. The composite structure is specifically optimized so that it has a high absorbance (emittance) in the far-infrared atmospheric window and nearly no loss in the solar spectrum. The calculated emittance is well reproduced in the experiment by our AAO sample. The fabricated porous membrane shows a potential cooling power density of 64 W/m(2) at ambient temperature (humidity similar to 70%) under direct sunlight irradiance (AM1.5). Experimentally, the sample is cooled by a 2.6 degrees C temperature reduction below the ambient air temperature in the sunlight. This performance shows little variance at night. The AAO approach proposed here may provide a promising way to produce low-cost and efficient radiative cooler in large scales for feasible energy conservation.