Abstract: Infrared radiation from the molten pool contains plenty of welding status information including the characteristics of the seam deviation.Infrared images of the molten pool were captured by an infrared sensitive high-speed camera during high power (10kW) fiber laser butt joint welding of 304 austenitic stainless steel with micro-gap seam (seam gap width was less than 0.1mm).By analyzing the molten pool characteristics,the information of seam deviation was explored.A keyhole formed when the laser beam was focused on a weldment and the metal vaporized instantly.Features of the keyhole infrared images,especially the characteristics of conjunction between the solid and liquid zones with unstable burr edge of seam were studied.A corner point detection method was proposed to detect the microgap seam deviation in high power laser welding process.Experimental results showed that the dense distribution center of the corner points of a molten pool infrared image had a close relationship with the weld seam deviation.The micro-gap weld seam deviation status in high power fiber laser welding can be determined by the corner point distribution density.