The effect of the temperature and microstructure on the fatigue crack growth behaviors of two microstucturally different AISI type 347 stainless steel(SS)s are studied at reactor operating temperature conditions. From the results, it was observed that the fatigue crack growth rate(FCGR) of type 347 SS increased as the test temperature was increased. However, its temperature sensitivity was reduced when the ΔK was large. The measured FCGRs for both TP347-LP and TP347-HP at 25oC and 345oC were lower than those presented in the ASME code. Especially near the threshold region, the trend curve in ASME code overestimated the FCGR of type 347 SS when compared with the obtained data. Even though the effect of the microstructure on the macro-crack growth rate was not significant during stage 2, the differences between threshold values for the specimen with a high precipitate density and the one with a low precipitate density were observed. It is assumed that these differences in the precipitate density, which resulted in a difference in mean free path length of dislocation movement, are the source of the difference in ΔKth of type 347 steels.