Abstract: Ultraviolet emission signals and infrared emission signals and audible sound signals were collected in CO₂ laser welding of 1.8 mm galvanized steel plate. The correlation and difference among three signals were compared between laser deep penetration welding and laser heat conduction welding. The transient behaviors of keyhole and plasma were also analyzed during laser penetration welding process. The results indicate that the keyhole and plasma are not always generated at the same time and the plasma is produced prior to keyhole. The plasma may appear in the heat conduction welding. Under the experimental conditions, the plasma forms after 3KG-*5-KG-*34 ms of laser being irradiated on the samples, while the stable keyhole forms at about 14 ms after welding. It has been found that the power intensity used to produce the plasma is lower than that of the keyhole.