K-constant fatigue crack growth (FCG) tests were conducted on low carbon steels with uniformly distributed hard particle with different of pearlite, bainite and martensite in a soft ferrite matrix. The FCG tests by using a single edge cracked tension (SECT) type was performed inside a scanning electron microscope chamber equipped with a servo-hydraulic fatigue machine. During the test, in-situ crack path observation was carried out to identify the crack tip stress shielding phenomena. From the results, influence of hardness of second phase particle on the FCG behavior in Paris regime was systematically investigated. The results revealed that the ferrite-martensite (FM) steel showed significantly higher FCG resistance compared to that of ferrite-pearlite (FP) and ferrite-bainite (FB) steels. The harder second phase particle would be more difficult to be plastically deformed, which would induce higher plastic constrain. This higher plastic constrain may result in significant crack closure effect and stress shielding effect, thereby increasing higher FCG resistance.