The output power of a frequency stabilized He Ne laser depends strongly on the cavity losses. In this paper, the additional losses which come mainly from Brewster windows plate sealing are quantitatively analysed. Using indium sealing techniques, the mount quality and precision for Brewster window are greatly improved, and the output power of lasers are apparently enhanced due to reduction of the cavity losses. As an example, a He Ne 633 nm frequency stabilized laser with cavity length of 110 mm can give a output power of 2.4～3.0 mW.