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Beam combination characteristics were numerically and experimentally investigated in an array laser amplification using stimulated Brillouin scattering (SBS) phase conjugation. To clarify the effect of piston errors due to imperfect phase locking, spatial intensity profiles of the beam combination output were calculated in both the near and the far field on the basis of the Rayleigh–Sommerfeld diffraction theory. The analyses indicate that piston errors are seriously detrimental to the quality of a beam combination output and should be eliminated by a proper phase locking. To estimate the relationship between the output beam brightness and partial phase locking due to back-seeding, a statistical calculation with a root-mean-square (RMS) criterion method was used. Enhanced brightness was obtained with only partial phase locking. Basic experimental results are also presented and show the same tendency to numerical results.