Accurate lymph node size analysis is important medically. This paper presents an improved fast marching method to perform semi-automatic segmentation for lymph node from CT images. In this work, we have incorporated the gray scale information of the target region into the fast marching speed term and have given a hard constraint for the stop criteria, instead of only using the spatial image gradient, to remedy the 'boundary leaking' problem of the traditional fast marching method. Various experimental results are provided to demonstrate the effectiveness of the proposed method.