The maximization of boron effect to stabilize martensitic microstructure in the vicinity of prior austenite grain boundaries has been investigated for advanced 9Cr-3W base steel in order to improve long-term creep strength at 650degreesC. Creep tests were carried out at 650degreesC for up to about 3x10(4) h. FE-AES analysis shows that the boron content in M23C6 carbides near prior-austenite grain boundaries is higher than that inside the grains. However, at a combination of high tungsten of 3wt% and high boron exceeding 90ppm, greater part of boron added is present as undissolved large particles of tungsten-rich borides after conventional normalizing at 1050degreesC (1323 K). The borides are substantially dissolved by raising the normalizing temperature to 1150degreesC (1423 K). The large dissolution of boron in the matrix causes large enrichment of boron in the M23C6 carbides. This significantly improves the long-term creep strength of the present steel. It is concluded that the amount of dissolved boron is essential to stabilize martensitic microstructure and that the high-temperature normalizing can increase it.