A commercial Al-Mg-Si alloy (Al 6082) was deformed by Equal-Channel Angular Pressing (ECAP) to produce bulk ultrafine-grained micro structure. The crystallite size distribution and the characteristic parameters of the dislocation structure were investigated by X-ray diffraction profile analysis. It was found that the crystallite size decreased and the dislocation density increased during ECAP deformation. The increase of the yield stress of the alloy was related to the increase of the dislocation density using the Taylor model.