Highly dense (Li,Na,K)NbO3 ceramics were prepared by normal sintering at reduced temperatures by adding excessive Na, and its phase structure and electrical properties were investigated as a function of the compensated Na content. A high piezoelectric coefficient d33 up to 279 pC/N was obtained at an optimized Na content, which was found to correspond to a phase boundary between two kinds of orthorhombic phases. The corresponding Curie temperature and electromechanical coupling factor reached 465°C and 46%, respectively. The phase boundary found in the present (Li,Na,K)NbO3 ceramics like the morphotropic phase boundary existing in the Pb(Zr,Ti)O3 system, but different from the polymorphic phase transition from an orthorhombic phase to a tetragonal one reported previously in most (Li,Na,K)NbO3 ceramics.