In this paper, the theoretical calculations of deff of GdCa4O(BO3)3 (GdCOB) along various phase-matching (PM) directions are reported. It is shown that the maximum deff is out of the main planes and the deff in the range of (0 < θ ≤ 90, 90 < φ ≤ 180) are larger than that in the range of (0 < θ ≤ 90, 0 ≤ φ ≤ 90). The PM direction along (θ = 66.8, φ = 132.6) for GdCOB crystal has the largest deff. Second-harmonic-generation (SHG) experiments were performed on GdCOB crystal along various PM directions. The results show that in GdCOB crystal, the intracavity SHG conversion efficiency is 27.6% along the PM direction (θ=66.8, φ=132.6) which is larger than that along PM directions in XY and XZ main planes, which are in good agreement with the theoretical calculations. The continuous-wave (cw) green laser output power is 1.22 W when LD pump power is 10 W in the Nd:YVO4/GdCOB laser. © 2000 Elsevier Science Ltd. All rights reserved.