The previous paper in this series describes probe beam application as a monitoring technique during YAG laser welding and clarifies the effectiveness of this method through rationalisation of the probe beam irradiation conditions during weld monitoring. During monitoring of the welding process, however, it is important to detect welding defects. Monitoring of defects in CO 2 laser welding has been previously reported in a handful of papers. 3 These efforts have to some extent seen practical applications for detection of perforations and underfills in welding of thin sheets. During detection of underfills in welding of thin sheets, however, the percentage of underfills is high in relation to the sheet thickness, and it remains unclear whether monitoring can be performed with much the same accuracy during welding of thick plates. For detection of defects in YAG laser welding, welding of thin sheets of around 1 mm thickness has been investigated, 6 but these studies focus on detection of the penetration depth and lap welding defects, whereas detection of underfills and misalignments in butt welding has so far been little documented. In the area of YAG laser welding of thick plates, detection tests during full penetration welding by bead-on-plate welding have been reported. For welding of thick plates, however, the literature contains few studies of continuous high-power laser welding and welding being performed by increasing the peak value of the laser power in pulsed welding. The purpose of this paper is to describe a technology for detection of defects during continuous high-power YAG laser welding of thick plates. Full and partial penetration detection tests are conducted by bead-onplate welding. Butt weld gap and misalignment detection tests are also conducted by butt welding. The changes in the monitoring signals then used to detect these defects are investigated, and a rational detection approach is proposed.