Abstract: The composition range of hydrogenated amorphous carbon films(a-C:H)is calculated theoretically in terms of the theories of fully constrainted networks(FCN).The theory predicts that the range of H,SP²C and SP³C for a-C:H is a special triangle in their ternary phase diagram.Plenty of data announces that the theoretical result is consistent with experimental one.A further conclusion is achieved that if the fractions of H and SP³C are given in a-C:H films,the others may be calculated theoretically.