This study aimed to assess creep crack growth rates on the base and welded metals of modified 9Cr-1Mo steel. For this purpose, welded specimens were prepared by the Shielded Metal Arc Weld method. To obtain mechanical properties concerning the base and welded metals, a series of creep and tensile tests were conducted at 600 °C, and creep crack growth tests were also performed under different applied loads using 1/2" compact tension specimens at 600 °C. Their creep crack growth rates were calculated using the empirical equation of the da/dt vs. C* parameter and compared. It appeared that, for a given value of C*, the rate of creep crack propagation was about 2.0 times faster in the welded metal than the base metal. This reason was that the welded metal was faster in the creep strain rate than the base metal.