Abstract: A method is presented in the paper to calculate Gini coefficient values of size distributions of cities by means of single logarithmed Lorenz curve when fractal structure has degenerated. First,determine the parameters A and B in the equation ∑kP(k)=A+Blnk through regression;Then compute Gini coefficient G value with the formula,G=(A-B+BlnN-50)/50,derived by the authors. Examples of calculation are given based on the date of urban population of Henan Province to show how to get G values in practice.