In order to research how to express formulae in network systems,FML (Formula Markup Language) based on XML (eXtensible Markup Language) is presented.At first,the relationship between FML and network,and connection and the difference between FML and MathML are analyzed.It is pointed out that VML or MathML is unfit for efficient transportation in networks and it cannot direct practical applications as a mature standard.And then,the grammar and usages of FML are described.At last,FMLViewer was presented,which is a powerful tool for explaining and displaying FML.Probing into FML and its usages has some value in both theory and practice for XML studies.