Dissociation behavior of methane ethane mixed gas hydrate coexisting structures I and II at constant temperatures less than 223 K was studied with use of powder X-ray diffraction and solid-state C-13 NMR techniques. The diffraction patterns at temperatures less than 203 K showed both structures I and Il simultaneously convert to 1h during the dissociation, but the diffraction pattern at temperatures greater than 208 K showed different dissociation behavior between structures I and II. Although the diffraction peaks from structure II decreased during measurement at constant temperatures greater than 208 K, those from structure I increased at the initial step of dissociation and then disappeared. This anomalous behavior of the methane-ethane mixed gas hydrate coexisting structures 1 and 11 was examined by using the C-13 NMR technique. The C-13 NMR spectra revealed that the anomalous behavior results from the formation of ethane-rich structure I. The structure 1 hydrate formation was associated with the dissociation rate of the initial methane-ethane mixed gas hydrate.