We report on the structural properties of natural gas hydrate crystals from the Sea of Okhotsk. Using powder X-ray diffraction (PXRD), it was determined that sediments from four locations contained type I gas hydrate, which encage mostly methane (96-98%) and a small amount of carbon dioxide. For all hydrates, the lattice constant was estimated to be 11.85 angstrom at 113 K, which approximately equals that of pure methane hydrate. The result is in good agreement with the structure of artificially synthesized methane + carbon dioxide mixed-gas hydrates. These results suggest that the lattice constant of the natural gas hydrate does not change due to a change of CO2 gas content. In addition, the thermal expansion of the sampled hydrate was measured for the temperature range of 83-173 K, and the resulting density of the hydrate crystal at 273 K was estimated to be 0.92 g/cm(3). These results are essential for applying natural gas hydrates as an alternative natural fuel resources. (c) 2005 Elsevier Ltd. All rights reserved.