Because of the lack of research, there is no mature design method for less-teeth gear transmission nowadays. In this paper, based on the features of less-teeth gear transmission, we presented the five main limitations of the modification coefficient for less-teeth gear and its relevant equations. By using these equations, taking z1=4, z2=54 as an example, we plot the closed graph for the modification coefficient on computer, and then analyzed it. By using this method, we can plot the closed graph of modification coefficient for any transmission ratios. Determining the modification coefficient according to its closed graph is not only quite easy to understand, but also more reasonable.