The coexistence of superconductivity and magnetism in Ho 1-x Dy x Ni 2 B 2 C is studied by using a phenomenological model that is based on the Ginzburg-Landau theory. This model includes two magnetic and two superconducting order parameters accounting for the multiband structure of Ho 1-x Dy x Ni 2 B 2 C. The doping dependence of T c and the effects of antiferromagnetic and spiral order in this material are analyzed.KEYWORDS: Ginzburg-Landau theory, Superconductivity, Antiferromagnetic order, Spiral order,The series of nickel-borocarbide, RNi 2 B 2 C (R = Y, Yb, Dy, Ho, Er, Tm and Tb) 1),2) , is a kind of magnetic superconductor.3) This class of materials shows a variety of magnetic ordered states, depending on the rare-earth atoms R.4) Among them, HoNi 2 B 2 C and DyNi 2 B 2 C constitute ideal materials to study the relation between superconductivity and magnetism because they display the same antiferromagnetic order, but have different values of the Néel temperature T N . Experiments show that simple de Gennes scaling of T c , which is expected to monotonically decrease with increasing (g J -1) 2 J (J + 1) according to the Abrikosov-Gor'kov theory, 5) does not exist for the alloy Ho 1-x Dy x Ni 2 B 2 C. (Fig. 1) HoNi 2 B 2 C and DyNi 2 B 2 C have antiferromagnetic (AF) order below T N = 5 and 10 K, respectively 6) . The spins order ferromagnetically within the a-b plane and antiferromagnetically in the c direction. For HoNi 2 B 2 C, a spiral (SP) short-range correlation exists i.e., the spins are aligned ferromagnetically in the a-b plane and are rotated by ~ 163° between adjacent planes. According to the band calculation for LuNi 2 B 2 C )7) , the Ni(3d) band makes the largest contribution to the density of states at the Fermi surface, suggesting that it is dominant in the formation of the superconducting state. However, other bands involving hybridization with B(2p), C(2p), and Ho(5d) or Dy(5d) should be included in the superconductivity. Hence, the superconductivity of nickel-borocarbide materials should be described by using a multiband model.For the AF order, the influence of the rare-earth magnetic moments is canceled for the Ni sites located exactly in the center of a tetrahedron of nearest Ho(Dy) atoms. The band originating from Ni(3d) no longer feels the magnetic moment of Ho(Dy) below the Néel temperature. On the other hand, the field generated by the moments of Ho(Dy) is not canceled at sites of other elements. In the case of SP correlation, the magnetic field at the Ni site is not canceled exactly. The SP phase also affects the superconductivity of the Ni(3d) band. For other ions, the effect of the SP correlation is not so important because the dominant effect essentially comes from the AF order. A simple phenomenological model 8) with two superconducting order parameters,