  We introduce a novel level crossing in the mass mixing between two QCD
axions, one canonical QCD axion and one $Z_{\mathcal N}$ QCD axion. The level
crossing can take place at the QCD phase transition critical temperature or
slightly before it, depending on the ratio of the axion decay constants
$\sim1.69$. The cosmological evolution of the mass eigenvalues in these two
cases is similar, however, the transition of axion energy density is completely
different. Finally, we estimate the relic density of the QCD axion dark matter.
This level crossing may also have some cosmological implications.
