In this paper, highly Yb-doped Ca4GdO(BO3)(3) (Yb:GdCOB) crystals were grown by the Czochralski method. The absorption and fluorescence spectra were measured. The laser of Yb:GdCOB crystal was demonstrated with different output mirrors when it was pumped by a fiber-coupled laser diode (LD) at the wavelength of 976.4 nm. The lowest pumping threshold was 280 mW, the highest light-light conversion efficiency was 33.8%, and the highest slope efficiency up to 70% was thus calculated from the linear part of the curve. The self-frequency doubling (SFD) green laser output was also observed.