Activated carbons with large adsorption capacity and high mesopore fraction were obtained from rice straw by a combination of pre-pyrolysis and NaOH activation procedures. The experiments varied the pre-pyrolysis procedure, impregnation ratio of activating agent, and activation temperature. Samples were examined by inductively coupled plasma-mass spectrometry (ICP-MS), X-ray diffractometry (XRD), field-emission scanning-electronic microscopy (FE-SEM), thermogravimetric analysis (TGA), and N2-adsorption analysis. The surface area and pore characteristics of the activated carbons were investigated by the Brunauer, Emmett, Teller (BET) method, Dubinin-Raduch (DR) model, and the t-plot method. The surface area, pore volume, and iodine adsorption capacity of the samples increased with increasing activation temperature and the impregnation ratio of the activating agent. A maximum surface area of 2093 m2/g was obtained at the activation temperature of 900 °C. The pore structure in the one-stage activation procedure was mainly microporous. Two-stage activation procedure efficiently enhanced mesopore volume, and therefore further increased the adsorption capacity of activated carbons. NaOH acted as both the activating agent in the reaction and as the cleaner to remove the ash. The results of this experiment will be useful in developing resource recovery systems from agricultural biomass.