Fluctuation of light intensity from the keyhole becomes more significant in full penetration welding than partial penetration welding, since the plasma produced in the keyhole can escape from the rear side of the keyhole. The plasma optical radiation emitted during Nd:YAG laser welding of S45C steel samples has been detected with a Photodiode and analyzed under different process conditions. As the results, the BOP was performed for welding, behavior of plasma, spatter or plume was monitored to determine the reference signal. Then, random combination was made for comparison with the reference signal, which aimed at verifying reliability of the welding monitoring system that this study intended to develop.