The effects of fine particle peening on the fatigue properties of transformation-induced plasticity (TRIP)-aided bainitic ferrite steel were investigated for applications to precision gears. Fine particle peening increased the fatigue limit and lowered the notch-sensitivity in the TRIP-aided bainitic ferrite (TBF) steel, compared with SNCM420 steel. In the stress intensity range of less than 25 MPa m^1/2, TBF steel subjected to fine particle peening exhibited a larger threshold value of the stress intensity range and higher crack propagation rate than SNCM420 steel. The increased fatigue properties are principally associated with (1) higher Vickers hardness, (2) higher compressive residual stress and (3) plastic relaxation resulting from the strain-induced transformation of metastable retained austenite in a surface hardening layer, which may suppress fatigue crack initiation and propagation.