Software estimation is an important prerequisite for project development management. The complexity of the software, the lack of historical experience, the incompleteness of estimating tools as well as human errors, often lead to software project estimation results far from the reality. In order to improve the estimation results, from a management point of view, the key estimation points in common software development model and the corresponding methods which can be used, are designated in light of the software life-cycle theory and COCOMO Ⅱ model, and what basic essences estimator should have are proposed and their positions in a typical software organization are pointed out. Finally, an application example is given to illustrate how to apply these measuring points by using a free software tool provided by the USC.