The α-phase in 60/40 brass added Si was investigated by transmission electron microscope (TEM) in order to understand the relationship between the ability of hardening during annealing and phase decomposition. The ability of hardening ( Δ HV), which is the value of difference between the maximum hardness during annealing and the value of as-quenched sample, was obtained measurement from the alloy. As a result, the Si-bearing alloy is higher in ΔHV than a base alloy without Si. The α-phase in the Si-bearing alloy showed fringes in its TEM image like as stacking faults, and extra reflected spots in its selected area diffraction pattern (SADP), which is corresponding to the 9R structure. Streaks were also observed together with spots in its SADP. Extra reflected spots and streaks in SADP became weak or disappeared when the annealing time increased. After 600 ks annealing, SADP obtained from the α-phase could be indexed as FCC lattice. Change in chemical composition of α-phase was also measured by the energy dispersive X-ray spectroscopy (EDS).