Abstract: The confocal laser scanning microscope(CLSM) combined with an infrared furnace was used to in-situ observe the carbide precipitation and sensitization behavior of AISI 302, AISI 304 and AISI 316L three austenite stainless steels with different carbon contents, the relationship between carbon content, heating temperature and time and carbide precipitation and sensitization band width was studied, and the grain boundary carbide precipitation appearing time-temperature curve and sensitization band appearing time-temperature curve were obtained. The results show that AISI 302, AISI 304 stainless steels would appear grain boundary carbide precipitation and sensitization behavior when they were annealed at 600～1 000 ℃, and carbide deposition and sensitization behavior could be observed more clearly and earlier on the grain boundaries of AISI 302 stainless steel with higher carbon content compared to AISI 304 stainless steel with lower carbon content. When the annealing temperature raised to 1 100 ℃, the grain boundary carbide precipitation became to dissolve and the sensitization band disappeared. Grain boundary carbide precipitation and sensitization behavior were not observed in AISI 316L stainless steel because of its lower carbon content. The sensitization band width increased with the increase of annealing temperature and carbon content, and it increased first and then decreased with the increase of annealing time.