In the design of high strength steels, bainite structures gain importance because of their excellent balance of strength and toughness. However, bainite still remains the least understood austenite decomposition reaction. Following the philosophy of Integrated Computational Materials Engineering (ICME), we combine various simulation and advanced characterization approaches at different length scales in order to improve the scientific understanding of this solid phase transformation. In the present work, bainite transformation in 100Cr6 steel with nano-sized cementite (theta) precipitation within bainitic ferrite (alpha(B)) is analyzed. The bainite transformation was introduced and investigated by TEM, atom probe tomography (APT), phase-field simulations and ab initio calculations. APT shows that in lower bainite isothermally held at 260 degrees C both epsilon and theta precipitate adopting plate-like shapes and precipitate under para-equilibrium mode. With the help of atom probe concentration data input and para-equilibrium phase diagram calculation using ThermoCalc software, isothermal bainite formation was simulated by means of phase field approach. To simulate the very fine cementite, ultra-small grid spacing, i.e. 2 nm, was applied in the simulation. A faceted model has been coupled in the simulation for both bainitic ferrite (alpha(B)) and cementite (theta) growth. APT showed the high amount of carbon trapped within bainitic ferrite. It is of significant importance to take into account both carbon partitioning and carbon trapping within bainite ferrite in bainite transformation simulations.