  By an explicit calculation of the continuum limit of the triangle graph
amplitude in lattice QED we show that in the axial Ward identity the ABJ
anomaly exactly cancels the pseudoscalar density term in the limit of infinite
fermion mass $m$. The result, a reflection of decoupling of the heavy fermion,
provides a convenient framework for computing the flavor-singlet or U(1) axial
anomaly in non-Abelian gauge theories on lattice. Our calculations on the
lattice are performed using Wilson fermions but the results are general.
