Abstract: To manage iterative development of incremental software so that resource allocation and cost estimating can be reasonably finished in incremental software development,the man-hour of development must be measured.Incremental requirements in each phase are implemented according to stated probability which is regarded as task accomplishment rate,a measurement method of iteration periods in incremental software development is provided.Furthermore,the man-hour of development for whole software project can be forecasted.The method is demonstrated with an example.