The strengthening mechanism of the large straining CP Al processed by rolling driven-ECA (Equal Channel Angular) was investigated. A theoretical calculation based on XRD analysis and Taylor equation indicates that the dislocation density of the large straining CP Al by rolling driven-ECA is very low. Low angle grain boundary and high angle grain boundary were measured by a crystal micro area orientation analysis technique and the results demonstrate that the low angle grain boundary is much more. Strengthening mechanisms of the large straining CP Al were quantitatively calculated based on the Hall-Petch relationships, and it is concluded that the strengthening mainly comes from the low angle grain boundary