Abstract: A mult-i scale diffusion algorithm is proposed for the halting speed fields （HSF） in geometrical active contours （GAC） model.By the algorithm,the object boundaries,homogenous regions and transition regions of the HSF are first defined through the introduction of two control parameters,and their values are set according to different complexities of the images.Then,the HSF is diffused by the mult-i scale diffusion algorithm and the diffused HSF is applied to the image segmentation based on GAC model.Experimental results on a synthetic image and two nature images demonstrate that the proposed algorithm can reduce both segmentation time and boundary leaking,in comparison with GAC model with the original HSF.