To develop ultra high-strength steels for automotive impact safety parts and forging parts, the fatigue properties of ultra high-strength low alloy TRIP-aided steels with martensite (TM), bainitic ferrite (TBF) and martensite-bainite (TBM) matrix were investigated. TM steels achieved both high tensile strength of 1370 MPa grade and excellent fatigue properties compared with TBF and TBM steels. This was caused by the suppression of crack initiations and propagations due to the effective transformation induced plasticity of retained austenite because of the existence of fine marteniste lath matrix and interlath retained austenite. TM and TBM steels indicated excellent fatigue properties compared with TBF steels. This was caused by the fine and uniform lath matrix and interlath retained austenite and increased tensile strength and yield strength due to the austempering treatment at low tempertature. Fatigue properties of ultra high-strength low alloy TRIP-aided steels were increased by the addition of nitrogen. Particularly, the nitrogen addition of 100 ppm improved fatigue properties. This suggested that initiation and propagation of cracks were suppressed by the effective strain induced plasticity of fine and stable retained austenite.