Active contours, as a technique for boundary extraction, have been successfully used in image processing and computer vision. One of the knotty problems of active contours is to conform to the object boundary with complex shape, which could bring heavy manual work at the initialization procedure. The gradient vector flow (GVF) field has been one of the most popular external forces that can increase the capture range of active contours and bidirectionally evolve the active contours toward the object boundary. However, it has a poor performance when dealing with some complex shapes, such as semi-closed concave, screwy concave, hooked concave, as well as the others presented in our experiments. We propose a novel GVF-based balloon force, which can efficiently assist the GVF field in driving active contours toward the complex object shapes. This additional force is used only when the active contours are prevented from evolving toward the object boundary by the saddle and/or stationary points in the GVF field. Therefore, it can maintain the bidirectional evolution property of the GVF and meanwhile take advantage of the power of the balloon force in segmenting complex shapes. Various experimental results on image segmentation are presented to show the good performance of the proposed active contour model that uses the GVF field and the proposed balloon force together.