BMS symmetry, which is the asymptotic symmetry at null infinity of flat
spacetime, is an important input for flat holography. In this paper, we give a holographic
calculation of entanglement entropy and Rényi entropy in three dimensional Einstein gravity
and Topologically Massive Gravity. The geometric picture for the entanglement entropy
is the length of a spacelike geodesic which is connected to the interval at null infinity by
two null geodesics. The spacelike geodesic is the fixed points of replica symmetry, and the
null geodesics are along the modular flow. Our strategy is to first reformulate the Rindler
method for calculating entanglement entropy in a general setup, and apply it for BMS
invariant field theories, and finally extend the calculation to the bulk.