Klystron microwave amplifiers play a vital role in addressing the increasing demands of high‐average microwave power for strategic applications such as linear accelerators, active denial technologies, radar, and so forth. Typically, klystrons have an efficiency of 50%‐60% that demands an efficient thermal design for dissipating the unused DC power in the form of spent electron beam in collector. Hence, thermal modeling of the collector for efficient heat dissipation is highly critical in design of high average power klystrons. Of several types of design, grooved collector design is widely employed so as to increase the surface area between the collector and coolant and thereby enhance heat transfer. In this article, a mathematical model and design strategy have been demonstrated to obtain the optimum dimensions, that is, height, depth, and width of fins based on film coefficient and Reynolds number. For validation, the dimensions are then simulated in a computational fluid dynamics software (ANSYS‐Fluent) demonstrating excellent agreement with the mathematical modeling. In addition, the optimum choice of grooving method (longitudinal or crossed) for the given power level has also been provided. The demonstrated strategy can also potentially be employed to other devices, which uses groove based design with water as coolant medium such as gyrotrons, plasma devices, and so forth.