To investigate the effects of precipitation, solid-solution, and dislocation strengthening on fatigue strength, fatigue tests were conducted on a series of ultrafine ferrite-cementite steels, the ferrite grains of which were smaller than 1 lm. To investigate the effect of precipitation strengthening by carbon addition, which increased the number of cementite particles in the ultrafine ferrite-cementite steel, several steels containing 0.15 pct (in mass pct) carbon and others containing 0.45 pct were studied. The effect of solid-solution strengthening was investigated by adding 0.1 pct phosphorus to some steels. Fatigue tests were also conducted on annealed versions of the ultrafine ferrite-cementite steels, to investigate the effect of dislocation strengthening, since the as-rolled versions were work hardened, due to warm-caliber rolling to refine the ferrite grains. The resultant tensile strength of the lab-prepared ultrafine ferritecementite steels ranged from 721 to 1048 MPa. All the lab-prepared ultrafine ferrite-cementite steels showed high fatigue-limit ratios (fatigue-limit/tensile strength) exceeding 0.5. The fatigue strength of the ultrafine ferrite-cementite steel was higher than that of ferrite-pearlite steel, and matched that of tempered martensite steel. In conclusion, all of these strengthening mechanisms satisfactorily improved the fatigue strength of the ultrafine ferrite-cementite steels. The ultrafine ferrite-cementite steel showed stable fatigue strength, due to its microstructure being both ultrafine and uniform.