GdxY1-xCa4O(BO3)(3) (GdYCOB) crystal has considerable potential as a noncritically phase-matched third-harmonic generator (THG) of Nd :YAG lasers. However, photo-induced damages like photo-refractive damage and gray-track occurred during the operation of high-power THG. In this paper, the effect of growth atmosphere on the threshold of photo-induced damage (gray-track) is examined. From our result, the gray-track of GdYCOB, grown in Ar atmosphere has a threshold of 2.3-3.1 MW/cm(2) (355 nm, 6 ns, 10 Hz). The gray-track threshold was increased to 8.4-10 MW/cm(2) when GdYCOB was grown in air ambient.