Keyhole formation is one of the most important issues in laser welding. Keyhole formation in meltrun welding of pure titanium at laser powers from 4 to 6 kW and welding speeds 100 mm/s was investigated by X-ray in-situ observation and numerical calculation using particle method considering Fresnel absorption and vapor recoil pressure. E−ects of concentrated energy absorption due to local laser beam reflection and thermal transportation due to melt flow on keyhole formation were studied by the numerical calculation using particle method. It shows that expansion of the keyhole tip was caused by multi-reflection of the incident laser beam. Moreover, the calculated vaporization rate indicates the energy consumption in forming or maintaining a keyhole.