  In this article, we study the implications of the coupling between
Axion-Like-Particles (ALPs) and Leptons to cosmology, in particular, the Big
Bang Nucleosynthesis (BBN). We show that the BBN, through the constraint on the
effective number of relativistic neutrino species, provides the most stringent
bound on the ALP-electron interaction strength for the mass of axion between 20
keV and 1 MeV. For other values of the mass, the BBN bound complements the
stellar-evolution and laboratory bounds.
