It has been reported that formation of Mo(Al,Si)2 phases by the addition of aluminum can improve the mechanical and oxidation properties of MoSi2, however, the constancy of the Mo(Al,Si)2 phase was not explored. In this study, MoSi2 preforms were added with Al whose contents were 1, 5, 10wt%, respectively, and these performs were sintered at 1150°C-1550°C, respectively. These MoSi2 specimens with the Al addition were heat-treated at 1350°C-1550°C in N2 atmosphere. Results showed that single Mo(Al,Si)2 phase existed only when the aluminum content was above 5 wt%. It was difficult to form single Mo(Al,Si)2 phase for the samples with 1wt% Al addition. For the samples with 5wt% Al, only the single Mo(Al,Si)2 phase was formed at 1350°C; and a large number of the tetragonal MoSi2 were re-appeared at 1450°C, 1550°C. For the samples with 10wt% Al addition, the single Mo(Al,Si)2 phase would always formed at individual sintering temperatures. The formation temperature of the single Mo(Al,Si)2 phase was decreased with increment of aluminum content. For the samples composed of the single Mo(Al,Si)2 phase, the Mo(Al,Si)2 phase disappeared gradually and was substituted for MoSi2 and a little of other phases, such as Mo3Al4Si2, by heat treatment at temperatures mentioned above for 40 min. This indicated that the Mo(Al,Si)2 phase was unstable at the temperatures above 1350°C.