A 200 watts excimer laser with excellent energy stability has been developed. The laser is based on the XMR model 5300, and thorough improvements were made over the XMR5300. The laser can produce a stable output of 200 watts (667 mJ, 300 Hz) under servo-controlled operation over 24 hours (greater than 25 million pulses). During this operation, the output fluctuation was found to be less than 1.5% for (sigma) and 10% for peak to peak. Horizontal and vertical beam divergences were less than 1 mrad and 4 mrad, respectively.