Crack Energy Density (CED) is the parameter that enables a unified description of crack behavior without any restriction on constitutive equation and its applicability has been shown for ordinary materials. In this paper, the concept of CED is extended to piezoelectric body and its fundamental matters are studied. The definitions of CED and its mechanical and electrical parts are given first and their path independent expressions are derived through the energy conservation law. Subsequently, the loading path dependence of mechanical and electrical parts even for reversible process is made clear. Moreover, some quantities related to CED are defined and their path independent expressions are given.