Gas hydrate pellets contains large amount of gas about 160–170 times as much as its volume and it is stored at 253 K by self-preservation effect. The self-preservation effect is exhibited by ice film on the gas hydrate surface. Phase contrast X-ray imaging is effective tool to distinguish ice from gas hydrate. It revealed that self-preserved gas hydrate pellets had ice film on its surface without internal dissociation by phase contrast X-ray imaging. On the other hand, gas hydrate pellets including NaCl were dissociated from its inside, then gas volume that they hold decreased.