Object oriented design and development are well-liked and accepted conceptions in today's software development circumstances. Object oriented design supports design principal such as inheritance, coupling, cohesion and encapsulation. The paper will deliver a method for reliability estimation of object oriented design in respect of functionality perspective. To design and deliver quality products within time and budget functionality plays a vital job. Functionality and reliability quantification approach is presented in this paper. This paper appreciates the need and importance of functionality at design phase and the statistical conclusion establishes functionality as an influencing factor for software reliability.