Authors proposed a creep life assessment method for boiler pipes using a small sample, where it is assumed that the creep property of each base metal in actual pipes is approximately constant in the thickness direction of the pipes. In this work, the assumption was examined for five pipes of modified 9Cr-1Mo steel, which had been in long-term use in USC power stations. Material properties such as hardness and creep life in the thickness direction of the pipes were investigated in detail. No difference was observed for those, except in the area less than about 0.4mm from the outer surface of the pipes, which means that the assumption in the assessment method is valid except in this area. Considering growth of the area during service time at the stations and the heat affected zone during electrical discharge machining, it is suggested that the creep life characteristics of the piping base material can be evaluated by collecting the small sample with a thickness of 2mm.