The performance of manufacturing systems can be adjusted by allocation buffer into the manufacturing system. Buffer will improve the performance of manufacturing systems by improving the utilization of the constraints; yet buffer will also increase the makespan and the work in progress. Due to the complex nature of the systems, buffer allocation is usually difficult to optimize. This paper presents a prediction model of the effect of buffer based on the shifting bottleneck detection and a blocking and starving analysis. The prediction model is used to optimize the buffer allocation using only a single simulation.