The lath martensite structure in steel offers high strength with a complex substructure, and its strength increases with carbon content. However, the mechanism of carbon strengthening is yet to be elucidated. In this study, we evaluate the tensile properties of as-quenched lath martensite without retained austenite in Fe–18Ni alloys containing 4–570 ppm carbon. In the 4 mass ppm carbon alloy, whose carbon is almost trapped by Ti(CN) particles, the work hardening behavior during uniform elongation is constant regardless of the size of the effective grain surrounded by a high angle boundary. In contrast, the yield point (YP), 0.2% proof stress (σ_0.2%), 0.6% proof stress (σ_0.6%), and maximum tensile strength (TS) in 7, 110, and 570 mass ppm carbon alloys increase with the refinement of their effective grain, consistent with the Hall–Petch relationship. The Hall–Petch intercepts for the YP, σ_0.2%, σ_0.6%, and TS are constant and unaffected by the carbon content. This suggests that the non-occurrence of solution hardening by solute carbon atoms in the lath martensite. The Hall–Petch coefficients for the YP, σ_0.2%, σ_0.6%, and TS increase with carbon content and are proportional to the square root of the carbon content. This indicates that the increase in carbon content increases the strength of the lath martensite via the refinement of effective grains and the increase in the effectiveness of grain refinement.