An all-solid-state single frequency Nd:YVO4/KTP laser with high efficiency and high power is reported, with fiber-coupled laser diode (LD) end-pumping Nd:YVO4 crystal, ring resonator consisting of four mirrors, and KTP crystal intracavity frequency doubling. Based on the frequency-doubling theory of elliptical Gaussian beam under conditions of critical phase mathcing, an improved intracavity frequency-doubling conversion efficiency is obtained by tuning the orientation of Nd:YVO4 crystal and choosing appropriate polarizing direction for fundermental frequency light to make the walk-off plane of KTP crystal paralle to the sagittal plane of cavity. The peak output power of single frequency green laser reaches to 4.8 W and the optical-optical conversion efficiency equals to 25.5%, when the pumping power is 20 W. In comparison, when the walk-off plane is perpendicular to the sagittal plane, the conversion efficiency is 21.8%, and the maximal single frequency green laser output power is 4.1 W.