We studied the melt composition to obtain pit free and highly Bi-substituted (BiLu)3Fe5O12 crystals by the LPE method. First, we confirmed that B2O3 doping into the melt increases Ko(Bi), and found that the decrease of R4 increase the gradient of growth rate dependence of Keff(Bi)/Ko(Bi). Secondly, we found that a mirror-like surface of the crystal is obtained by Gd2O3 doping into the melt. Using the melt with Gd2O3/Lu2O3≥1, the pits, which conventionally appear in LPE growth of (BiLu)3Fe5O12, were suppressed and pit-free and highly Bi-substituted (BiLuGd)3Fe5O12 crystals obtained.We measured the optical properties of 250 μm thick (BiLuGd)3Fe5O12 crystal in order to evaluate its applicablity to an optical isolator for a 1.3 μm wavelength range. A sufficient properties of Faraday rotation θF = −1,800 deg/cm and the extinction ratio −40dB were obtained.