The substitution phenomenon in the (Tb,Lu)(3)Al5O12 mixed garnet crystals have been studied by solidification of corresponding stoichiometric melts using micro-pulling-down apparatus. By substitution of Tb3+ with Lu3+, th, tendency of TbAlO3 perovskite phase formation in undoped Tb3Al5O12 decreases. Although growth of mixed crystals in general does not allow uniform distribution of constituents in the resulting material, the invariant point was found to exist in the system studied in the vicinity of Tb2.2Lu0.8Al5O12 composition. Thus, the uniform single-crystal fibers of the above materials up to 500 mm in length and about 1.3 mm in diameter were produced at a pulling-down rate of 1.2 mm/min.