Records from 56 stations on free-field within 150km to causative fault surface are used as database during the Mw7.9 Wenchuan earthquake in china. According to this study, the duration of ground motion is strongly affected by rupture distance, site conditions, fault rupture directory and thrust of hanging wall. Duration is obtained based on the time between 10 and 80 per cent of Arias intensity. A simple attenuation relation in terms of rupture distance is proposed and compared with some available predictions. The durations of tri-component ground motions on hanging wall are smaller than those on the foot wall in the same range of rupture distance. Durations of ground motions close to rupture fault resulting from directivity effects are significantly different between the forward and backward direction, and this difference reaches nearly 2 times on average.