  The supertranslation ambiguity of angular momentum is a long-standing problem
in general relativity. In the context of post-Minkowskian expansion, the
supertranslation ambiguity arises the puzzle of the angular momentum loss to
start at ${\cal O}(G^2)$ or at ${\cal O}(G^3)$ in gravitational scattering. In
this paper, we propose a generic prescription to fix the supertranslation
ambiguity at the linear order in post-Minkowskian expansion which will uniquely
determine the angular momentum loss. The proposal is self-contained in the
post-Minkowskian expansion and involves only null infinity data.
