Influence of boron concentration in CoFeB on the transport properties of CoFeB (B 20% and B 16%)/MgO/CoFeB magnetic tunnel junction (MTJ) was investigated. Boron distribution was studied by using X-ray photoelectron spectroscopy (XPS). High-resolution transmission electron microscope was utilized for analysis of the texture and interface quality. The MTJ with the boron diluted CoFeB (B 16%) pinned layer shows 10% higher MR than the CoFeB (B 20%). HRTEM shows that the MgO/CoFeB interface for the sample with diluted CoFeB layer has a better epitaxial MgO/CoFeB growth. Higher boron concentration in the as-deposited CoFeB stays high after annealing but boron content in the MgO increases as well. These results suggest that that MR improvement is directly related to the distribution of the boron in CoFeB/MgO/CoFeB MTJs after annealing.