Effective software estimate is the most challenging and the most important activity in a software development project management. Only when using scientific methods to reasonably and reliably estimate the scale, the amount of work and cost of the software project, do the people implement good plan and control. The paper introduces several typical ways of software cost estimate, deeply analyses the model of Putnam, uses the examples to research the scale, the number of labors at the peak and the time of the project to provide information for managers to make positive decisions.