The microstructure of a monolithic Ll2Al67Mn8Ti25, compound changes significantly when modified with a small amount ofNb (0. 2~4. 0at%) and with corresponding adjustments to the Ti and Al contents. In 32 experimental alloys near the Al3Ti-8Mncomposition region of the Al-Ti-8Mn-Nb quaternary system, five multiphase regions. Ll2+Al2Ti, Ll2+DO22, Ll2+Ll0, Ll2,+Al2Ti+DO22 and Ll2+Al2Ti+Ll0 formed adjacent to the original Ll2 single phase. When Nb content keeps constant, the plate-like or nettyAl2Ti phase precipitates appear with the decrease of Al/Ti ratio. The ripple-like Ll0 AlTi phase may appear in the alloys containedhigher Nb c0ntents. A room temperature constitutional diagram near the Al3Ti-8Mn region of Al-Ti-Nb-8Mn quaternary system hasbeen determined.