Cu/Zn beta-phase alloy (beta-brass) colloid was successfully synthesized by reducing the organometallic complexes in the organic liquid phase. It was confirmed by X-ray diffraction that the material formed pure P-phase. The morphology for the beta-brass particles was observed using transmission electron microscopy (TEM). The dispersion of the particle diameter was almost single modal, and the diameter was estimated at 8 +/- 2 nm. A clear peak around 520 nm was seen in the optical absorption spectrum for the P-brass colloid, and it is attributed to the surface plasmon absorption. We also simulated the optical absorption spectrum using the bulk dielectric constant for beta-brass. The simulation qualitatively reproduced the experimental data well.