Rule curves of reservoir are necessary guides for long term reservoir operation. The objective of this research is to improve the rule curves of small reservoir in rural area in Thailand which struggles with floods and draughts caused by climate conditions. The Kaeng Loeng Chan reservoir was considered to improve their rule curves. The calculations are based on a simulation model using Genetic Algorithms and the water balance equation (GAs-WB) with the objective function of a minimal average annual water shortage. The results have shown that the pattern of the obtained rule curves similar to the existing rule curve. The obtained rule curves were used to simulate the Kaeng Loeng Chan system for evaluating the situation of water deficit and flood release in long term operation considering synthetic inflow as well as increased inflow of 10%, 20% and 30%. The results have indicated that the situations of water shortage and excess release of the new rule curves are smaller than theirs existing rule curves.