The effect of initial microstructure, martensite, tempered martensite, bainite and ferrite+pearlite, on long term creep strength property up to about 80000h at 848K has been investigated on a 0.5Cr-0.5Mo low alloy ferritic steel. No significant effect of tempering after quenching on the long term creep strength has been observed. However, strong effect of transformation process during cooling from austenite has been observed. Creep strength of bainite microstructure is higher than those of martensitic microstructure, independent of tempering after quenching. In the low stress condition in which applied stress is lower than proof stress, creep strength of full annealed microstructure which consists of ferrite and pearlite has been found to be higher than the others. The smallest creep rate of a full annealed microstructure in the long term creep region should be caused by both very low dislocation density of the initial microstructure and small amount of dislocations generated under the low stress condition.