Active contours, known as snakes, have been widely used for medical images. In this paper a new algorithm for 3D segmentation is proposed based on the snake technique, which can include both gradient and textural features as image forces and a new external force to improve the sensitivity of a normal snake against image noise. After the contour has been well defined on one slice of a volume data by a 2D segmentation method, the contours of the subsequent slices can be obtained through our modified snake method. Experiments on medical volume images have proved the validity of the algorithm.