The paper proposes an approach for setting the size of project buffer based on the theory of unascertained rational numbers and critical chain management. This method applies the optimistic time into programming schedule plan and using the addition operation of unascertained rational numbers to obtain the unascertained schedule plan, to finish and generate cumulative reliability. This technique is applied to calculate the buffer time by using the optimistic construction period and total project period when finishing reliability is 90%. This feasibility of the model has been proved by an example. © (2012) Trans Tech Publications, Switzerland.