Mumford and Shah’s variational model in the 2-phase piecewise constant case is very efficient in image segmentation.However

if the original image is contaminated by some noise

the level set method for solving the model is very sensitive to the initial level set function and the parameter of the length of the evolving contour.Here we propose a two-step segmentation method

where in the first step

a coarse segmentation is obtained by using some traditional method

and in the second step

the coarse segmentation is used as an initial solution in the variational model.Moreover

we gave a model for adaptively estimating the parameter of the length of the evolving contour

where the parameter is defined as an increasing function of the variance of the noise.Combination of the two-step segmentation and the adaptive estimation model not only enables automatic evolution but also ensures fast and accurate partition.Experiment on some computer- produced images and real images shows that the algorithm proposed here is very efficient.