Three morphology CeO2 supports with three-dimensionally ordered mesoporous structure (3DOM), nanotubes (NT) and nanocubes (NC) were synthesized by nanocasting of a mesoporous silica KIT-6 template with cubic Ia3d symmetries, hydrothermal method with and without urea, respectively, and supported cobalt catalysts were investigated on the production of COx-free hydrogen from ammonia decomposition for 300-600ºC. Morphological structure of CeO2 significantly influenced the catalytic activities in the ammonia decomposition, and the Co/CeO2-3DOM catalyst had a higher catalytic performance than Co/CeO2-NC and Co/CeO2-NT. It was elucidated by the formation of smaller particles size and more oxygen vacancies. The Co/CeO2-3DOM catalyst exhibited good stability and reusability at 600ºC for ammonia decomposition due to the three-dimensionally ordered mesoporous structure as anchoring cobalt active sites and prevention from active site aggregation. A mechanistic insight has been presented to understand the structure-activity relationship based on the structure of the CeO2 supports.