Poly(allylamine)-stabilized spherical and rod-shaped copper nanoparticles were synthesized by a simple chemical reaction. The synthesis was performed by the reduction of copper(Ⅱ) salt with hydrazine in aqueous solution under atmospheric air in the presence of poly(allylamine) (PAAm) capping agent. Besides providing long-term stability to the nanoparticles by preventing particle agglomeration, polymer capping agents such as PAAm make the particles dispersible in aqueous solution. Noteworthy advantages of the synthetic method include its production of water dispersible nanoparticles at room temperature without inert atmosphere, making the synthesis more environmentally friendly. The resulting copper nanoparticles were investigated by UV-Vis spectroscopy and transmission electron microscopy. The authors found that several factors, including the amount of NaOH solution, concentration of PAAm, and reaction time, affect the composition, size, morphology, and degree of agglomeration of the resulting copper nanoparticles. The amount of NaOH in the reaction is crucial for the synthesis to result in either pure copper or copper oxide-containing copper nanoparticles as well as to produce the highest possible yield of copper nanoparticles. In addition, the reaction time and concentration of PAAm play key roles in controlling the size and shape of the nanoparticles, respectively. The resulting colloidal copper nanoparticles exhibit large surface-enhanced Raman spectroscopy (SERS) signals.