Abstract—A measuring maintainability of software system in the design phase, may help a software designer must improves the maintainability of software system before deliver to a customer. In this paper presents a multivariate linear regression for establish the maintainability estimation model in terms of flexibility and extendibility are sub-characteristics of maintainability as criteria to evaluate maintainability model of class diagram and develop the Maintainability Estimation Tool (MET) for a maintainability estimation of class diagram. This tool help a software designer for improves the maintainability of class diagram in design phase and help reduces the increasing high cost of software maintenance phase.