The kinetics of isothermal bainite formation at 483 K, 503 K and 533 K in a bearing SAE 52100 steel (1,01% C - 1,36% Cr - 0,32% Mn - 0,25% Si (wt.%)) is investigated theoretically and experimentally. It is found that the bainite formation becomes significantly slower with decreasing temperature. To interpret the results, a physical model with a concept of reconstructive approach proposed in the literature is applied. The model gives a reasonable agreement with experimental results, although a certain deviation exists at the late stage of the transformation. This deviation is attributed to the diffusion coefficient used in the model without considering the effect of alloying elements, mainly chromium.