Detailed studies of high resolution TEM inages on the modulated structure caused by Zn vacancy ordering along [110] direction in BZT sintered at $1400^{circ}C$ for 90 hours had revealed that the images which had hexagonal patterns were similar to those obtained from the structure which had no modulation, These images had appeared over the wide ranges from -30 nm to -42 nm in defocus values and from 2 nm to 20 nm in thickness. The computer simulated images showed that the modulation due to Zn vacancy ordering had made a small change in contrast in the interior of hexagonal pattern, which was very difficult to differenciate in experiments. The image which demonstrated the modulated structure very well was the one which obtained at -52 nm in defocus value and 16 nm in thickness.