Abstract: Using labyrinth weirs for operating reservoirs at higher than normal stages can expand reservoir capacity and flood storage and improve power generation efficiency, while keeping the same levels of inflow, dam height, weir height, and flood control standards. This study has adopted a nonlinear programming method to examine the relationship of weir height versus sidewall angle of labyrinth weirs at fixed reservoir stages on the basis of weir capacity analysis. We have developed and solved an optimization model, and its objective function is maximizing the weir crest elevation under the constraint of normal discharges that ensure safety and the constraint of stream-wise weir size not longer than spillway control section. The results show that to maintain normal ranges of discharge capacity at fixed reservoir stages, weir height can be reduced if sidewall angle increases. Applying this technique to optimization of weir design, we found out that if dam height is determined by normal water level, the optimized weir is able to lower check water level or design water level and hence improve flood control standard. If dam height is determined by either check water level or design water level, using such weirs can expand reservoir capacity and ensure flood discharge safety.