This paper presents biochar application derived from agricultural by–products for methylene blue (MB) adsorption in an aqueous solution. Biochar formed from agricultural by–product biomass has a large specific surface area (SSA), and high content of elemental components, including carbon (C), hydrogen (H), oxygen (O), and nitrogen (N). The color adsorption capacity has achieved a good signal for biochar application to environmental treatment. Biochar samples from rice straw pyrolysis at 400℃ were selected to investigate the adsorption capacity of methylene blue. The findings demonstrated that the adsorption capacity increased and peaked at the concentration of methylene blue at 200 mg/L. The fine–sized biochar (biochar 212 µm) achieved the best adsorption efficiency for methylene blue with a saturation adsorption threshold of 6.3 mg/g. Methylene blue adsorption capacity can reach > 75% efficiency. The reason can be explained by the advantage of large specific surface area, the diversity of pore size systems inside the biochar structure, and the biochar surface that can provide critical functional groups such as –OH, C=O. Thus, research on biochar application derived from agricultural by–products in polluted water treatment indicates their valuable potential in the future.