The lack of knowledge of the relation between a lidar backscatter signal and particle size makes it challenging to retrieve ice-cloud particle size from spaceborne lidar observations. This study employs a synergistic combination of the state-of-the-art invariant imbedding T-matrix method and the physical geometric-optics method (PGOM) to investigate the relation between the ice-crystal scattering phase function at 180° (P11(180°)) and particle size (L) for typical ice-crystal shapes. In particular, the P11(180°) -L relation is quantitatively analyzed. The dependence of the P11(180°) -L relation on particle shape can be used with spaceborne lidar observations to detect ice-cloud particle shapes.