The scatter in creep-rupture data and the accuracy of prediction by TTP (Time-temperature parameter) method for long-term creep rupture strength have been examined for 16 types of ferritic heatresisting steels. The logarithm of long-term creep rupture time was observed to be normally distributed. The TTP of Larson-Miller (LMP) and Manson-Haferd (MHP) was used for prediction of long-term creep rupture strength. The constants of TTP (C=20 for LMP, Ta=450 and Ya=14.55 for MHP), which are the average values of the optimized constants for each heat, were applied. An excellent prediction of long-term creep rupture strength whose RMS (root mean square) value is less than 0.3 was achieved for 95% of 87 heats of a wide variety of low alloy steels from 0.5Mo steel to 5Cr-0.5Mo steel.