Recent researches on the behavior of gravelly sands advocate for the use of skeleton void ratio to characterize their density state. Skeleton void ratio corresponds to the void ratio of grains constituting the stress-bearing skeleton. However, such a void ratio relies on parameters difficult to determine in practice, such as the fraction of fine grains that take part actively in the load bearing skeleton. Also, it fails to consider the effect of Grain Size Distribution (GSD) of gravel and sand grains. Therefore, the skeleton void ratio index introduced by Chang et al (2015) is revisited to account for the effect of GSD of both gravel and sand grains. Two semi-empirical equations are developed in this paper to connect GSD parameters with skeleton void ratio parameters. The validity of the proposed equations has been checked for a particular class of gravelly sand materials. A series of specially-designed drained triaxial tests on gravelly sands were then conducted. Test results show that it is essential to consider the effect of GSD when using skeleton void ratio index. It also verifies the effectiveness and applicability of the proposed updated skeleton void ratio, which shows advantages in characterizing critical state lines of gravelly sands.