We employed Na beta-alumina ceramics as a candidate of structural material of which strength can be controllable. Mechanical strength of Na beta-alumina ceramics revealed to be enhanced up to 50% by ion exchange of Na+ by larger K+. Maximum mechanical strength was as large as 450 MPa that can be easily degraded by simple annealing at moderate temperature. Such large strength maintained on service could make it useful as structural material. Furthermore it is advantageous for recycling because it can be easily broken after annealing. The increase in mechanical strength on ion exchange was not dependent on the composition of beta-alumina but dependent on the internal stress developed due to the difference in the cation sizes. After showing maximum strength, prolonged ion exchange time leads to strength degradation due to the crack generation. When NaK-beta-alumina ceramics was used instead of Na beta-alumina, ion exchange time showing maximum strength and degree of strength degradation had been changed. (c) 2006 Elsevier B.V. All rights reserved.