Background: The geometry features of a protein surface plays key role in protein-protein interactions.Various descriptors or methods have been developed to identify convex and concave regions in proteins, such as protrusion index, surface roughness, FADE, CX and VisGrid.However, these methods are either coarse descriptors or involve intensive computation.Prompted by the CX and VisGrid algorithm, an improved surface structural parameter called the convex-and-concave index(CCI) was developed to identify convex and concave regions on protein surfaces.