A new donor-acceptor method and a calculation technique for estimating the surface tension force term have been developed in order to improve VOF (volume of fluid) method. The improved VOF consists of (1) calculation of liquid quantity transferred from donor to acceptor cell by grouped into 11 interface patterns, (2) correction of liquid transfer rate by considering movement in oblique direction, (3) correction of inner cells with defects under conservation of total liquid quantity, (4) highly accurate calculation technique for estimating the surface tension force term by approximating the liquid-vapor interface to a circle. We applied the improved VOF method to a simulation of oscillating liquid drop in which the surface tension force is dominant and the obtained frequency agrees well with the theoretical one.