A new approach to a positronium lifetime calculation is proposed. The method is completely relativistic and gauge invariant. The full positronium width is determined through several parameters connected with a dynamics of forming the bound state. These parameters play the same role as structure functions in the case of inclusive deep-inelastic lepton-hadron scattering or the pion decay constant [ital f][sub [pi]] for strong interactions. They should be calculated using a nonperturbative model of the bound state. The explicit calculations of (para)positronium lifetime to one-loop order in [alpha] are presented.