  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 AdS_5/Z_k to AdS_3/Z_k case, and we proposed that the
AdS_3/Z_k decays into AdS_3/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 AdS_5 orbifold. The
decay of AdS_5/Z_k is dual to the transition between the dual gauge theory
vacua on R_t x S^3/Z_k, parametrized by different holonomies along the
orbifolded spatial cycle. We constructed the instanton solutions describing the
transitions by making use of instanton solutions on R_t x S^2.
