Measuring software maintainability early in the development life cycle, especially at the design phase, may help designers to incorporate required enhancement and corrections for improving maintainability of the final software. This paper developed a multivariate linear model 'Maintainability Estimation Model for Object-Oriented software in Design phase' (MEMOOD), which estimates the maintainability of class diagrams in terms of their understandability and modifiability. While, in order to quantify class diagram's understandability and modifiability the paper further developed two more multivariate models. These two models use design level object-oriented metrics, to quantify understandability and modifiability of class diagram. Such early quantification of maintainability provides an opportunity to improve the maintainability of class diagram and consequently the maintainability of final software. All the three models have been validated through appropriate statistical measures and contextual interpretation has been drawn.