The title compound, poly[diamminehexa-mu-cyano-dicopper(I)copper(II)mercury(II)], [Cu3Hg(CN)6(NH3)2]n, has a novel threefold-interpenetrating structure of three-dimensional frameworks. This three-dimensional framework consists of two-dimensional network Cu3(CN)4(NH3)2 complexes and rod-like Hg(CN)2 complexes. The two-dimensional network complex contains trigonal-planar Cu(I) (site symmetry m) and octahedral Cu(II) (site symmetry 2/m) in a 2:1 ratio. Two types of cyanide group form bridges between three coordination sites of Cu(I) and two equatorial sites of Cu(II) to form a two-dimensional structure with large hexagonal windows. One type of CN- group is disordered across a center of inversion, while the other resides on the mirror plane. Two NH3 molecules (site symmetry 2) are located in the hexagonal windows and coordinate to the remaining equatorial sites of Cu(II). Both N atoms of the rod-like Hg(CN)2 group (Hg site symmetry 2/m and CN- site symmetry m) coordinate to the axial sites of Cu(II). This linkage completes the three-dimensional framework and penetrates two hexagonal windows of two two-dimensional network complexes to form the threefold-interpenetrating structure.