Phase transformation and precipitation hardening behavior of the water atomized copper alloy powder was studied by aging treatment, to develop high strength Cu-40Zn-X (X: Cr, Fe, Sn) alloys by powder metallurgy process. Super-saturated solid solution elements of Cr and Fe are formed in the brass matrix, and single beta phase was retained in the raw powder after water atomization. Solid solubility of Cr and Fe decreased with increase of aging temperature, and phase transformation evolved from single beta phase to alpha + beta duplex phase structure after aged at the elevated temperature of 773 K and over. It was clarified that Cr showed higher precipitation potential than Fe in the brass matrix. The hardness depended strongly upon solid solubility of Cr and Fe, and upon phase transformation.