A gradation equation with one parameter is derived to describe continuous gradation of coarse-grained soil. The basic properties of the gradation equation are studied

and the ability of the gradation equation to reflect two typical gradation curves of continuous gradation is analyzed. The applicability of the gradation equation is verified by several gradation curves of coarse-grained soil used in earth-rock dam engineering

and the range of ? deduced for the gradation is well represented. The study shows that the gradation equation can reflect the two typical hyperbolic and sigmoidal

gradation curves. The range of gradation parameter is 0–1. the curve gradation equation expresses is hyperbolic for small value of the parameter

and is sigmoidal for large value of the parameter. The gradation equation widely applies for gradations of different coarse-grained soils used in earth-rock dam

and it is more practical than fractal gradation equation and continuous gradation equation. The equation parameter greater than 0.13 implies well representation of the gradation

which provides a basis for the gradation design of coarse-grained soil for earth-rock dam.