Flux-cored arc welding (FCAW) is relatively a new process for joining of modified 9Cr-1Mo (P91) steel. In this study, effect of shielding gas composition, inclusion content, gas tungsten-arc welding (GTAW) surface remelting, and postweld heat treatment (PWHT) on toughness were investigated. The high amount of silicon resulted in the formation of δ-ferrite in basic flux-cored weld. A mixture of 80% argon + 20% (80A) carbon dioxide shielding gas during welding resulted in the required toughness of 47 J at room temperature. The 95% argon + 5% carbon dioxide (95A) gas-shielded welds have lower toughness due to higher amount of δ-ferrite (4%) than 80% argon + 20% carbon dioxide welds (2%). In essence, most desirable shielding gas medium to achieve optimum toughness was 80% argon + 20% carbon dioxide in basic flux-cored arc welding. © 2010 ASM International.