In this study, the preservation ability of natural gas hydrate (NGH) with different particle size from 0.50 mm to 30 mm was investigated. It is known that natural gas preservation does not occur when NGHs are in powder form and their size is under several Lens of micrometers, while the dense bulk NGH was preserved after the initial rapid dissociation, it is important to investigate the required size of NGH particle for self-preservation to occur when NGH is used as a storage media of natural gases. Despite the difference in particle size, the experimental results herein revealed that the NGH sample was enveloped by an ice film caused by the initial rapid dissociation. The thickness of the ice film grown during the initial rapid dissociation was almost the same and did not depend on the particle size of NGH being over 1.0 mm and higher than that of methane hydrate. The thickness of the ice film necessary for the self-preservation of NGH may be the reason why powder NGH does not show the self-preservation phenomenon. However, we revealed that NGH particles with diameters of more than 0.50 mm were preserved for two weeks. (C) 2014 Elsevier Ltd. All rights reserved.