Rupture life is a main property for a material using at high-temperature condition. Usually, the rupture life is gained from creep rupture test. As creep and stress relaxation are two main behaviors for a material served in high-temperature environment, it is important to work out the interrelationship through which one of the two behaviors can be deduced from the other one. Recently, a number of researchs have taken stress relaxation test to replace creep rupture test on studying the creep behavior, and furthermore predicting the rupture life and the stress relaxation test is proved to be superior to the traditional creep rupture test for its short time, small at damage, abundant of information and so on. In this work, the stress relaxation test was used to analyze the creep behavior of two HR3C heat resistant steels with different grain sizes. Additionally, considering the change of microstructure during serve period, the aged HR3C steel was used to compare with as-received HR3C steel for studying the aging effects on the creep behavior. Furthermore, the creep behavior was correlated to their microstructure characteristics. The result was shown that the creep behaviors of two HR3C heat resistant steels varied significantly in spite of their similarity in chemical composition. The coarse grained HR3C steel had lower creep rate, larger stress exponent, greater activation energy and higher creep resistance than that of fine grained HR3C steel for both as-received one and aged one. The long-term aging process damaged the microstructures of two HR3C steels, increased aged HR3C steel's creep rate, lowered stress exponent and activation energy and reduced creep resistance. And the damaging effects on the coarse grained HR3C steel were larger than that on fine grained HR3C steel, which meant the coarse grained HR3C steel had much more stable creep resistance than that of fine grained HR3C steel.