Software effort estimation is an important activity for effective project management. To predict the required effort in the early phase of the software project, this paper proposes(1)a WBS analysis process and method to estimate project effort from high level WBS and(2)an modified effort estimation model by utilizing four task assignment factors, team size, concurrency, intensity, fragmentation and one new factor, the WBS effort. In the present study, these five factors are shown to improve the predictive ability of the original task assignment effort estimation model. For the data examined, our model is shown to have goodness of fit and quality of estimation superior to that of the original one.