High power lamp pumped YAG lasers of 6 kW and 10 kW average output power with good beam quality have been developed by using MOPA arrangement with multi-pump cavities for oscillator and amplifiers. For 6 kW laser system, efficient pump cavity for lessening thermally induced beam distortion effects has been designed, which realized output power greater than 700 W and M2 value less than 70. CW 3.5 kW has been realized from 5 cavity laser oscillator and CW 6.7 kW output power has been obtained by adding 5 cavity amplifier to the oscillator. During development, much amount of power of scattered and deformed light surrounding a main laser beam was observed for increased number of amplifiers. By applying spatial apertures between amplifiers, laser output beam quality has been improved to as good as M2 of 73 [beam parameter product (2 (omega) *2 (theta) ) :100 mm (DOT) mrad], enough for fiber (NA 0.15, (phi) 0.6 mm) delivery. Furthermore, 14-rod cavities, 10 kW Nd:YAG laser system, which can generate world-highest output power of 10 kW or more as a solid state laser, have been developed for thick plate processing with 137 mm (DOT) mrad (M2 approximately 101) beam parameter product. This laser system has achieved the transmission to 9 kW or more for (phi) 800 micrometer/NA 0.2 fiber.