Abstract: The bulk density of unconsolidated bottom sediments is critical to gas hydrate resource assessment. The study of this paper is to find a way to estimate the bulk density of unconsolidated sea-bottom sediments bearing gas hydrate, when there is no well logging or core data available. Core dada including bulk density, porosity and grain density from ODP164, ODP204 and IODP311 were used in this study. And the distribution of these physical properties and relationship among them were analyzed. Results demonstrate that porosity is very high in the unconsolidated sediments, and plays the most important role in determination of bulk density. Therefore, bulk density of unconsolidated marine sediments could be estimated from porosity when well data or core data were unavailable. Taking the gas hydrate-bearing sediments in the South China Sea as a case, the calculated bulk density fit well with the measured data.