Mobile learning is becoming a popular concern. WML, a major technical support for mobile learning, does not support the expression of mathematical formulas, so the mathematical formulas are difficult to transfer and reuse. This paper studies the structural characteristics of mathematical formulas and proposes a mathematical formula structure model. Using this model, a universal mathematical formula markup language is designed. This markup language may be used to transport the expression of mathematical formulas in a mobile learning environment. Its structure is simple and is easy to browse.