Abstract: A technique of laser welding glass GG17 was developed to get clear photographs of the keyhole for quantitative study. The geometrical characteristics of the keyholes are experimentally investigated. Basic on the known keyhole geometry, the Fresnel absorption and the conduction and convection heat flux loss of the keyhole wall were calculated arc compared. A concept of self-regulation of the front keyhole wall for getting energy balance is proposed. The major force that balances the surfactension is also discussed.