For infectious diseases, as highly pathogenic avian influenza virus H5N1 (A/H5N1 HP), the need for an early detection and warning mechanism is essential, especially when countries have been free of disease for an extended period of time. Evaluating the sensitivity and the expected cost of surveillance are necessary steps to ensure an efficient and sustainable system. Stochastic scenario tree modeling has been used here to assess the sensitivity of the A/H5N1 HP surveillance system in sector IV of Thailand. The whole process of disease detection, from passive surveillance to X-Ray, has been described and sensitivity of each component and of the overall system has been estimated. Scenarios, according to selection of high risk areas, inclusion of components or sampling procedure, have been tested and discussed.