A yellow continuous wave with beam quality M2 = 4.6 and output power of 4.8 W at 589nm is generated by intracavity sum-frequency mixing of 1064nm and 1319nm radiations of a Nd:YAG laser. To achieve high beam quality at high power, thermally near-unstable fiat-Sat resonators with two-rod birefringence compensation are designed to obtain the large fundamental mode size inside the Nd:YAG rods and the same beam width inside the KTP crystal. The optimal intracavity power ratio of both 1064 nm and 1319 nm beams is also considered. The output power fluctuation of the yellow laser remains less than 5% in four hours.