The magnetic and superconducting properties in a series of pseudo-quaternary compounds Y 1Ϫx Tb x Ni 2 B 2 C were investigated by the temperature-dependent magnetization M (T) for applied field parallel and perpendicular to the c axis and by in-plane resistivity, (T). There was a large anisotropy in M (T), which is attributed to the crystalline electric field effects. As the Tb concentration x is increased, the superconducting transition temperature T c decreases and disappears rapidly in the vicinity of xϭ0.4 while the Néel temperature T N appears abruptly near xϭ0.4 and increases with Tb content. In addition, the increase of M (T) at low temperatures below T N was observed for xу0.4, of which the onset temperature T WF is regarded as development of weak-ferromagnetic component. T WF was also in proportion to the Tb content x accompanied by T N . The linear decrease of T c for dilute Tb concentration seems to follow an Abrikosov-Gor'kov expectation, while the sudden destruction of T c near xϭ0.4 seems to have close correlation with the emergence of magnetic states, i.e., antiferromagnetic and weak-ferromagnetic states. Possible scenarios for the correlation are discussed.