Abstract: This paper provides a method of measuring genetic differentiation by analyzing gene regulation and expression. A Genetic Differentiation Value (Dg) was found, which could be used to determine the difference among different tissues. Using the method of the information theory, the authors found a function, that is the Gene Regulation Entropy (Hr), which could describe the extent of differentiation. The quantity of regulation information could be estimated from Hr·Dg has the property of intensity and Hr has the property of content. The value of Dg and Hr could be measured by isozymes or other proteins. In the end of the paper, how to use Dg and Hr in differentiation, biological evolution, biological information, the complexity of population and practical application in forest population genetics have been discussed.