High-resolution electron microscopy (HREM) has been used to demonstrate the structural characteristics : of ordered (Pb0.5Ca0.5) TiO3. The fi ne structure of both ordered and disordered domains was exposed in lattice level, It was shown that there are 1/2 {111}, 1/2 {110} and 1/2 {111} ordering coexisted in (Pb0.5Ca0.5) TiO3 and all of them are chemical ordering due to the layers of Pb2+ and Ca2+ cations align alternatively along different directions. For 1/2 {111} chemical ordering, a special type of boundary defined as "vernier boundary" was introduced.