Abstract: The microstructure, mechanical properties and residual life of Cr5Mo heat resisting steel and its weld joint taken from Pt-hydrofomer vessel pipeline which had been in oil refinery service for 14 years were studied. The experimental results showed that:The change in the microstructure and properties of base metal is little, the residual life was estimated for 100,000 hours. The mirostructure of overheated zone in HAZ of weld oint had been transformed from martensite to tempered troostite, the martensitelike structure on the weld metal side in the viinity of fusion line had been tempered oo. and a arbon-enriched zone appeared nearby the fusion line. The room temperature strength and high temperature strengh of weld join were higher than those of base metal, but the ductility and toughness were lower. The weld joint was a weak site susceptible to rak initiation and propagation, Intendance must be intensified during the operational period.