In order to determine the active mechanism of metakaolin(MK) as partial replacement of cement in concrete from the nanoscale structure perspective, nanoindentation characteristics of calcium hydroxide(CH)-MK blended with 5 M sodium hydroxide(NaOH) solutions simulating the condition of MK in concrete has been studied. The results indicate that three distinct phases are found by the nanoindentation and BSE tests, such as HD C-S-H gel, LD C-S-H gel and the reaction products with the network structure, the residual unreacted MK and CH, respectively. The volume fraction of HD C-S-H gel is about 96% of all reaction product phases. This means that the use of MK in concrete can increase the volume fraction of HD C-S-H gel and thus will improve the properties of concrete. In order to determine the active mechanism of metakaolin (MK) as partial replacement of cement in concrete from the nanoscale structure perspective, nanoindentation characteristics of calcium hydroxide (CH) -MK blended with 5 M sodium hydroxide (NaOH) solutions simulating the condition of MK in concrete has been studied. The results indicate that three distinct phases are found by the nanoindentation and BSE tests, such as HD CSH gel, LD CSH gel and the reaction products with the network structure, the residual unreacted MK and CH, respectively. This means that the use of MK in concrete can increase the volume fraction of HD CSH gel and thus will improve the properties of concrete.