A diversion weir is a headwork facility built across a river to raise the water level and to divert water for various purposes, such as irrigation, hydropower generation, etc. Diversion weirs with sidewise intakes are widely used in plain rivers. They are composed of many structural components which are designed for different purposes. In this thesis, a Windows-based, visual, user friendly program named WINDWEIR was developed in Visual Basic.NET programming language for the optimum design of a diversion weir with sidewise intake. It determines the overall dimensions of each of the components of the diversion weir and the total cost of the whole structure. It also performs stability analysis. It is such a flexible computer program that a design engineer can assess various dimensions of the structure from viewpoints of safety and economy by performing quick successive test runs to achieve an optimum solution among various alternatives.