Abstract: An investigation of detonation wave diffraction over a divergent nozzle was carried out experimentally and numerically. The experiment was conducted in a detonation tube with double exposure holographic interferometric measurement, which offers better resolution and quantitative information as well, in comparison with conventional flow visualization such as schilieren method. Adaptive finite volume method combined with kinetic chemical reaction model was programmed to simulate the flow field. The numerical results agree fairly well with the experimental measurements. It was found that detonation diffraction has quite a lot of flow feature compared with shock wave diffraction, including the detachment of reaction front and leading shock wave and reignition phenomena. The divergent angle and initial pressure have great influence to the flow field of detonation diffraction. The lower initial pressure, the earlier the detachment of chemical reaction front and leading shock wave will happen.