Microstructure, hardness and chemical composition of P91 steel weld metal were studied in aswelded state and after creeprupture test, while the gradient of carbon activity in white bands of fracture surface simulated using ThermoCalc software. Results show that significant depletion of alloying elements can be found at interbead boundary in aswelded state, featured by low hardness, carbide dissolution, low carbon mass fraction and recrystallization to ferrite grains, where creep cracks are easy to form. The carbon activity at interbead boundary is far above the surrounding matrix, and with the depletion of alloying elements up to the measured level, the carbon element in white bands will deplete to the mass fraction less than 0.01 %, when the carbides dissolve completely. The recrystallization of martensite to low hardness ferrite at interbead boundary during creep process will make its creep property greatly reduced.