Laser induced plasma is an important physical phenomena companied with laser keyhole welding. In this paper, the relation between weld penetration and optical signal was studied. Through many experiments, variety of optical signals were obtain when weld penetration was different. The results showed that the optical signals had better stability in full-penetration, and in partial- or non- penetration, optical signals had higher fluctuations. The aim of this study was to find a method to monitor and control weld penetration in laser welding.