Deformation characteristics of brass alloy are still under discussion, particularly concerning the critical level of when the change of deformation mechanism occurs. Previous research showed that the addition of Mn on brass alloys resulted in grain refinement and mechanical properties alteration. However, the effects of Mn on the deformation characteristic of brass alloys have not been investigated. In this research, Cu-Zn-xMn alloys were manufactured by gravity casting process using pure Cu and Zn ingots, as well as Mn chips as the feeding materials. Mn addition was varied to 1.26, 3.48, and 5.83 wt.%. As-cast samples were homogenized at 800 oC for 2 h in a muffle furnace. The samples were then cold-rolled with the level of deformation of 20, 40, and 70 %. Samples characterization includes chemical composition analysis, microstructure observation, tensile and hardness testing. The results showed that addition of Mn for 5.83 wt.% and above created β’ phase, which is richer in Mn compare to that in the matrix. This phase segregated in the grain and along the grain boundary with irregular forms. Significant increase in hardness, yield and tensile strengths was observed with addition of Mn. The maximum elongation was achieved by addition of 3.48 wt.% Mn, while further addition tended to decrease it. At 20 % deformation, slip dominated and its density reduced with addition of Mn. When the deformation level increased to 40 %, twinning replaced slip as the predominant mechanism. Twinning density is slightly increase with the presence of Mn. Further deformation at 70 % produced shear bands and flattened the β’ phase. Greater Mn content led to formation of more shear band.