The magnetic and superconducting properties in the single crystals of Er 1Ϫx Tb x Ni 2 B 2 C compounds are investigated by temperature-dependent magnetization M (T) and in-plane resistivity (T). It is found that the superconducting transition temperature is suppressed by adding Tb in the sites of Er and it follows well de Gennes scaling for both T c ϾT N and T c ϽT N . From the T c variation, the magnetic pair breaking strength of Tb is estimated to be similar to the Er. The antiferromagnetic and weak ferromagnetic transition temperatures show distinct behaviors in terms of the de Gennes factor in the Er-rich (xр0.4) and Tb-rich (xу0.6) side. These observations will be discussed under the consideration of magnetic ground states of ErNi 2 B 2 C and TbNi 2 B 2 C and their interactions.