Solar absorber is one of the important parts in a solar collector with a surface that must have high absorbance in the visible and near-infrared regions and low emission in the infrared region. The purpose of this study was to optimize the optical properties of anodizing of aluminum impregnated with nanoparticles for solar absorber coatings. The effects of key factors such as coating thickness, time and voltage of coloring process as well as the influence of heat treatment, zincating, ordering of the coating pores and multi-step anodizing was evaluated. Absorbing surfaces were prepared by anodizing in sulfuric and phosphoric acid followed by a black coloration process using Ni and Cu based baths and optimal conditions were selected. Coatings formed in phosphoric acid had less thickness and better selective optical properties (absorbance 0.9733 and emission 0.2539) than coatings in sulfuric acid. Scanning electron microscope (SEM) and optical microscope were employed for evaluation of structure and thickness of coatings. Optimal condition obtained at 12 V and 5 min for coloring. Effect of heat treatment on optical properties was investigated. To this end, aluminum substrate was annealed at 400 ?C in a furnace for 3 hour and as-coated samples were annealed at 400, 500 and 600 ?C for 1 hour. X-ray diffraction (XRD) was used for evaluating the effects of heat treatment on coating structure. Substrate annealing increased emission with no change in the value of absorbance. Heat treatment after coloring increased absorbance of the coating. Also emission was increased by annealing at 400 ?C but decreased after 500 and 600 ?C annealing. Zincate treatment was used by immersion in zincate solution for 30 seconds; results of energy dispersive spectroscopy (EDS) analysis and SEM images showed that zincate treatment before and after coloring decreased absorbance and emission of coatings. For ordering the pores, the layer created in one step anodization was removed by a mixture of H3PO4 (6 wt %) and Cr2O3 (1.8 wt %) and anodizing was carried out, again. Experimental findings showed that ordering of the pores decreased emission. In multi-step anodization, between the two stages of anodizing, the pores have been widened by 5 vol % phosphoric acid solution at 30 ?C. The results indicated that two-step anodization increased absorbance and emission and three-step process for low thickness coatings decreased emission.