Activation of mesoporous carbon CMK-3 with CO 2 for hydrogen storage was studied. Huge structure and texture changes emerged for the activated CMK-3 based on the characterization by using XRD, TEM and nitrogen adsorption at 77K. The ordered mesoporous structure of CMK-3 gradually became disorder and its specific surface area and volume of pores especially micropores were enhanced remarkably. Hydrogen sorption measurement showed that the activation led to an obvious increase of the H 2 sorption capacity of CMK-3. The maximum H 2 uptake of 2.27wt% at 77K and 1bar was obtained for the sample activated at 1223K for 8h. The small pores with the diameter smaller than 1nm contributed greatly to the H 2 uptake, and were confirmed more effective than other pores for hydrogen storage.