In this work, we study various decays of heavy B/D mesons into the a 1 (1260), based on the form factors derived in different nonperturbative or factorization approaches. These decay modes are helpful to explore the dynamics in the heavy to light transitions. Meanwhile they can also provide insights to a newly discovered state, the a 1 (1420) withAvailable theoretical explanations include tetraquark or rescattering effects due to a 1 (1260) decays. If the a 1 (1420) were induced by the rescattering, its production rates are completely determined by those of the a 1 (1260). Our numerical results for decays into the a 1 (1260) indicate that there is a promising prospect to study these decays on experiments including BES-III, LHCb, Babar, Belle, and CLEO-c, the forthcoming Super-KEKB factory and the under-design Circular Electron-Positron Collider.