Laboratory tests were performed on Toyoura sand specimens to investigate the relationship between degree of saturation S r , B-value and P-wave velocity V p . Different types of pore water (de-aired water or tap water) and pore gas (air or CO2 ) as well as different magnitudes of back pressure were used to achieve different Sr (or B-value). The measured relationship between B-value and V p was not consistent with the theoretical prediction. The measurement shows that the V p value in the specimen flushed with de-aired water is independent of B-value (or Sr ) and is always around the one in fully saturated condition. However, the V p value in the specimen flushed with tap water increases with B-value, but the shape of the relationship between V p and B-value is quite different from the theoretical prediction. The possible explanation for the discrepancy between laboratory measurement and theoretical prediction lies in that the air exists in the water as air bubbles and therefore the pore fluid (air-water mixture) is heterogeneous instead of homogenous assumed in the theoretical prediction. Laboratory tests were performed on Toyoura sand specimens to investigate the relationship between degree of saturation S r, B-value and P-wave velocity V p. Different types of pore water (de-aired water or tap water) and pore gas (air or CO2) as well as different magnitudes of back pressure were used to achieve different Sr (or B-value). The measured relationship between B-value and V p was not consistent with the theoretical prediction. The measurement shows that the V p value in the specimen flushed with de-aired water is independent of B-value (or Sr) and is always around the one in fully saturated condition. However, the V p value in the specimen flushed with tap water increases with B-value, but the the possible explanation for the discrepancy between laboratory measurement and theoretical prediction lies in that the air exists in the water as air bubbles and therefore the por e fluid (air-water mixture) is heterogeneous instead of homogenous assumed in the theoretical prediction.