Abstract: To solve the problem of direct observation of the keyhole during deep penetration welding, a modified "sandwich" specimen was proposed to directly observe the keyhole during deep penetration laser welding with a 10 kW fiber laser. A distinct keyhole wall and liquid motion along the wall were observed directly. The moving liquid "shelf" on the front keyhole wall and the accompanying hydrodynamic and vapor phenomena were observed simultaneously. Micro-droplets torn off the keyhole wall and the resultant bursts of vapor were also visualized. The emission spectrum inside the keyhole was captured accurately using a spectrometer to calculate the characteristics of the keyhole plasma plume. The experimental results suggest that the hydrodynamics on the keyhole wall has a dominant effect on the weld defects.