Bending rigidity (8f) of constituent filament in a single strand consisting the geometical structure of: 2- ply multifilament yarn was theoretically derived in terms of helix angle of single strand before plying(as), plying helix angle(oF) and the ratio of the distance to the filament from strand axis(Y) to strand radius(Rs) by strain energy method. And the change of bending rigidity of constituent filament was analyzed and discussed with change of the ratio of the distance to filament from strand axis to strand radius for the purpose of determining the plying twist processing condition.