A new 12%Cr steel integrating good creep properties, fabricability and corrosion resistance up to 650degreesC is being developed. For each step of this development, long term oxidation tests (about 6000h and 8000h) were made in pure water vapour in the temperature range 600-650degreesC. The laboratory and industrial heats were tested in comparison with two 12%Cr steels (X20CrMoV12-1), a 9%Cr steel (T91) and a fined grain austenitic stainless steel (TP347FG). Corrosion damage was measured using mass losses obtained after a reducing descaling process. Weight loss and metallographic results confirm the good corrosion resistance in steam of the new steel, VM12, and show either 2 different corrosion mechanisms or the same mechanism but with 2 different spreading rates: for the VM12, one X20 heat and TP347 steels, the time required for the whole surface of the samples being covered with corrosion products is definitely longer than for the other 9-12 % Cr.