Abstract. The characteristics of intracavity second-harmonic generation (SHG) at 1.06m in GdCa4O(BO3)3 (GdCOB) crystals cut for different type I phase-matching (PM) directions of (,)=(66.8, 132.3); (19.4, 0); (90, 46) have been investigated. It was found that the intracavity SHG was significantly efficient in the PM direction of (66.8, 132.3), and that the intrinsic lower effective nonlinear coefficient (deff) was responsible for the less-efficient SHG in the other two directions. A maximum CW SHG output power of 2.81W was obtained with an optical conversion efficiency of 18.7%, while the corresponding effective intracavity SHG efficiency was determined to be 41.3%. The intracavity SHG efficiency of GdCOB has been found to reach two-thirds of that obtained with type II phase-matching KTiOPO4 (KTP).