The authors have investigated aluminum and silicon alloying of Cr - Ni - Mo steels. Quantitative data on undercooling austenite decomposition under bainite transformation temperature range was obtained by magnetometric method. The Avrami equation was adopted for the kinetics of bainite transformation analysis. It was shown that the silicon addition reduces the temperature range of bainite transformation and increases the stability of undercooling austenite. However, long durations of holding may cause decomposition of undercooling austenite by perlitic mechanism. Aluminum in analyzed alloys moves the region of bainite transformation to higher temperatures, at the same time, the incubation period of bainite transformation reduces. Based on obtained data, it is recommended not to use aluminum alloying for massive billets because continuous cooling causes the decrease in despersity of bainite and reduces mechanical properties of bainite alloys.