Yttrium calcium oxyborate YCa4O(BO3)(3) (YCOB) is a novel non-linear optical crystal possessing good thermal, mechanical and nonlinear optical properties. Large-aperture YCOB crystals with 75 mm diameter were grown for high-average power frequency conversion on the mercury laser system. The growth morphology (included facet and spiral growth), cracking and inclusions in the as-grown crystal boule were discussed as the critical problem for large-aperture YCOB crystal growth. This can be minimized through modification of the growth program, including pulling rate, separation procedure. and cooling program. High-average power frequency conversion of the mercury laser using YCOB has been demonstrated, and experimental validation of YCOB material yields 50% conversion at 10 Hz has been achieved. (c) 2006 Elsevier B.V. All rights reserved.