The structure and magnetic properties ofNdMn2(Ge1−xSix)2compounds(x=0–1.0)have been investigated. All the compounds crystallize inThCr2Si2-type structure. Substitution of Si for Ge leads to a linear decrease of the lattice constants and the unit-cell volume. With increasing temperature, for the compounds withx⩽0.5,both spin reorientation and ferromagnetic– paramagnetic transition are observed as well atTrandTC(Mn),respectively. The spin reorientation temperatureTrand the Curie temperatureTC(Mn)decrease linearly with Si content from 215 and 339 K atx=0–40and 306 K atx=0.5,respectively. In compounds with0.3<x⩽0.5the ferromagnetic component and the antiferromagnetic component may coexit. For compounds withx⩾0.6the ferromagnetic–antiferromagnetic and the antiferromagnetic–paramagnetic transition are observed and the Curie temperatureTC(Nd)decreases monotonically while the Ne´el temperatureTN(Mn)increases monotonically with Si content. A tendentious spin phase diagram has been obtained. The saturation magnetization at 1.5 K decreases first, goes through a minimum atx=0.5,and then increases again. ©1997 American Institute of Physics.