Abstract: Objective To explore the necessity for the judgement of respiratory phase in localization and the correction of Z-axis coordinates under the therapy of gamma knife for hepatic cancer patients. Methods The motion range of right diaphramatic dome in eupnea and the position of middle point were observed. The position of middle point was marked in body surface using a steel ball with a diameter of 2 mm during CT scan of 5 mm slice thickness. Whether the position of diaphramatic dome and the middle point is in the same slice helped to ascertain the respiratory phase and correct the Z-axis coordinates. Results In the entire 122 patients， there were 38（31%） cases with the position of diaphramatic dome and the middle point in the same slice， 84 cases（69%） not in the same slice，48 cases（39%） with difference of one slice， 18 cases（15%） with difference of two slices， 12 cases（10%） with difference of three slices and 6 cases（5%） with difference of more than three slices. Conclusion The correction of Z-axis and the determination of PTV coordinates should not only refer to the range of respiratory range， but also to the respiratory phase.