(1) Institut Francais du Pétrole, 1 et 4, Avenue de Bois Preau, 92506 Rueil Malmaison Cedex, France.
            (2) Ecole des Mines de Saint-Etienne, SPIN, 158 Cours Fauriel, 42023 Saint-Etienne Cedex 2
 

The calculation of the refractive index of methane hydrate is carried out using a modified Lorenz-Lorentz model. The occupency of methane into the structure of hydrate is taken into account. It is calculated from the van der Waals and Platteuuw theory and shows small changes when the experimental pressure varies from 30 to 110 bats and temperature varies from -3 to +12°C. But we observed great changes in the relative index of refraction with the methane occupancy (the relative index of refraction is the ratio between the relatives index of refraction of hydrate phase and liquid phase) The relative index of refraction characterize the ability of the methane hydrate particles scatter light. Thus, the occupancy factor of methane must be taken into account when the calculation of size distribution uses the optical properties of particles.