[en] We study the condensation of localized closed string tachyons on AdS orbifolds both from the bulk and boundary theory viewpoints. We first extend the known results for AdS₅/Z_k to AdS₃/Z_k case, and we proposed that the AdS₃/Z_k decays into AdS₃/Z_k' with k^' < k. From the bulk viewpoint, we obtain a time-dependent gravity solution describing the decay of AdS orbifold numerically. From the dual gauge theory viewpoint, we calculated the Casimir energies of gauge theory vacua and it is found that their values are exactly the same as the masses of dual geometries, even though they are in different parameter regimes of 't Hooft coupling. We also consider AdS₅ orbifold. The decay of AdS₅/Z_k is dual to the transition between the vacua of dual gauge theory on R_t x S³/Z_k. We constructed the instanton solutions describing the transitions by making use of instanton solutions on R_t x S². (orig.)