In situ diagnostics of laser welding has been carried out using laser-induced breakdown spectroscopy (LIBS). The influence of weld pool areas (melt pool, solidified weld seam) on the laser plasma properties and LIBS measurements has been investigated. It is shown that the molten metal and high temperature of sample surface increase the LIBS plasma brightness as well as its temperature. The influence of the welding parameters on the LIBS signals has been studied in order to develop technique for on-line detection of nonoptimal laser welding conditions leading to the occurrence of defects in weld seams. An imperfect laser welding mode has been detected in situ for the first time in LIBS measurements.