The Brooks’ Law says that adding manpower to a late software project makes it later. Brooks developed the law through observation of many projects and derived the generalization. His explanation was quite reasonable and convincing. However, it becomes a debilitating statement to any software project manager who is faced with a late project. This paper presents an in-depth study of Brooks’ Law using the system dynamics simulation approach. Unlike the first order approximation of Brooks’ Law, we found some interesting results that can benefit the practicing project managers. We have found that adding people to a late project will always increase its cost, but the project may not always be late. Only under certain degree of sequential constraints among project tasks will the project be delayed. To investigate the impact of sequential constraint, we simulated numerous task conditions and found that there is a time line T for each project that if enough manpower is added before T, the project still can finish before the scheduled deadline. If manpower adjustment is after T, then the project will definitely be late. This way, the Brooks’ Law is revised and turned into a useful guiding principle to benefit software development.