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 d(33) 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 degrees 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)O-3 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.