Effort estimation of project at early stage is an important step that manager decide to take the job or not. The effort estimation not only affects the return of investment of the project, but the key indicator that the project leads to success or fail. This paper illustrates an approach of project estimation that was based on function requirement decomposition and expert judgment. This will increase the accuracy of estimation at the earliest software development stage. By using this approach, the manager not only gets more information about the effort of the project, but the key risk while developing project. This information facilitates the milestone, risk management, and project management planning.