Abstract: Now a days, the correct estimation of effort, cost and time for the process of software development plays a major role in the success or failure of software engineering projects such that the acquisition of project size is the first step to estimate the effort of software. COCOMO|| Model is the clearest and most reliable model for the estimation of the costs of software. Results from previous studies indicate that because of different structures in models, changes in the proposed hypotheses with the passage of time and different estimations in project size, the difference between predicted and real values are huge. In this study, an effective scheme is proposed which improves the time, cost and effort in the software for the COCOMO|| Model. The proposed scheme uses Object-Oriented Project Size (OOPS) metric to decrease the errors of source-code size estimations. The OOPS used here is extracted from the class diagram of the project. The obtained experimental results from 12 Java projects show that the proposed method satisfies the COCOMO|| Model properties as well and decreases the Mean Magnitude of Relative Error (MMRE) compared to the other works.