Image segmentation is treated as a key issue in image processing and machine vision,  and has been the bottleneck of development.  Transition region is a special region located between the object and background.  Having the aid of extraction of transition region to segment image is a kind of burgeoning technology.  Two kinds of methods have been introduced: methods based on gradient and methods based on non-gradient.  Briefly,  the extracted effects and existent problem have been analyzed.