Nuclear magnetic resonance measurements in hydrate-bearing sandstone samples from the Shenhu area, South China Sea were used to study the effect of gas hydrates on the sandstone permeability. The hydrate-bearing samples contain pore-filling hydrates. The data show that the pore-filling hydrates greatly affect the formation permeability while depending on many factors that also bear on permeability; furthermore, with increasing hydrate saturation, the formation permeability decreases. We used the Masuda model and an exponent N = 7.9718 to formulate the empirical equation that describes the relation between relative permeability and hydrate saturation for the Shenhu area samples.