The driving forces of two possible bainitic reactions within the bainitic transformation temperature range have been calculated for a CuZnAl alloy. The results show that ΔG β′→ α′ > 0 and ΔG β′→ β′ 1 + α < 0 in this transformation temperature range. Accordingly, the bainitic transformation can only proceed by the diffusional reaction β′ → β′ 1 + α. Furthermore, the driving force of 9R ordering transition of α has also been calculated. On account of ΔG α→ α′ < 0, the disordered α phase produced after the diffusional bainitic transformation may further transform into the ordered one with 9R structure.