CERN-LHC. Measurement of the dijet production cross section for jets containing a b-hadron (b-jets) in proton proton collisions at a centre-of-mass energy of 7 TeV, with 4.2 fb^-1 of data collected by the ATLAS detector in 2011. The cross section is measured for events with two identified b-jets with a transverse momentum pT > 20 GeV and a minimum separation in the eta-phi plane of Delta R = 0.4. At least one of the jets in the event is required to have pT > 270 GeV. The cross section is measured differentially as a function of the dijet invariant mass, dijet transverse momentum, boost of the dijet system, and of the rapidity difference, azimuthal angle and angular distance between the b-jets. Ranges of the variables of the measured differential cross sections: - m_bb : 50 - 1000 GeV (mass of the b-bbar di-jet system) - pT_bb : 0 - 400 GeV (transverse momentum of the b-bbar di-jet system) - y_B : 0 - 2.5 (boost of the b-bbar dijet system y_b = (y1+y2)/2 ) - Delta phi : 0 - pi (azimuthal angle between the two b-jets) - Delta R : 0.4 - 4.0 (angular distance between the two b-jets) - y* : 0 - 1.7 (rapidity difference between the two b-jets y* = |y1 -y2|/2 ) Cuts defining the fiducial region: - Leading jet pT > 270 GeV and |eta| < 3.2 - Both b-jets with pT > 20 GeV and |eta| < 2.5 - The two b-jets must have an angular separation Delta R > 0.4.