An effective way to synthesize activated microporous carbons with a high specific surface area is developed by single step reaction between Arundo donax and solid KOH at 600 °C for 2 h. The texture and the specific surface area of the activated microporous carbons can be controlled by adjustment of the ratio of Arundo donax and solid KOH. The prepared microporous carbons display larger surface areas and micropore volumes than those of the activated microporous carbon prepared by a two-step reaction. The porous carbon prepared with the KOH/biomass weight ratio of 2 (KLB2) exhibits the largest surface area of 1122 m2 g−1 and the highest micropore volume of 0.50 cm3 g−1. Among the materials studied, KLB2 exhibits the highest CO2 adsorption capacity at 273 K of up to 6.3 mmol g−1 at 1 bar while the adsorption capacity at 273 K is increased to 15.4 mmol g−1 at 30 bar. Present work demonstrates that highly stable activated microporous carbons can be prepared in a single step for effective CO2 capture for both post- and pre-combustion. As the highly stable microporous carbons are prepared from waste biomass, it can provide an efficient way for developing cost effective adsorbents for CO2 capture at low and high pressure.