Fine-scale bainitic microstructure with excellent mechanical properties has been achieved by transforming austenite to bainite at low temperature ranging from 200oC to 300oC. Microstructural observations and hardness measurements show that transformed microstructures consist of bainitic ferrite and carbon-enriched retained austenite. The thickness of bainitic ferrite plates is less than 50 nm. The hardness reaches approximately 640 HV1. Strong austenite and/or large driving force at the low transformation temperature leads to ultra fine bainitic ferrite plates. X-ray diffraction analysis indicates that low-temperature bainite transformation is an incomplete reaction. The carbon content in carbon-enriched retained austenite is below the para-equilibrium (Ae3′) phase boundary predicted. The carbon content in bainitic ferrite is less than that T0′ phase boundary predicted.