Abstract: The effects of strain aging on the microstructure evolution and internal friction of F40 hull structure steel were studied by SEM, TEM and multi-functional internal friction apparatus. The result showed that there was no obvious change in microstructure during the process of strain aging, displaying better mierostructure stability. A peak was found in the curve of internal friction to temperature, which can be attributed to C trapping on dislocations and dislocation motion. The strain aging phenomenon induced an increase in hardness associated with ductility loss and large toughness reduction. This trend is attributed to dislocation configurations and the interaction between interstitial carbon and dislocations.