Magnetic Fe₃O₄nanoparticles (NPs) were successfully synthesized via co-precipitation method using ferric chloride and ferrous sulphate as the starting materials. The shape and the size of Fe₃O₄NPs were controlled by using different types of additive including ammonium hydroxide and sodium hydroxide. The results revealed that by adding ammonium hydroxide, the particles attained a spherical shape with a uniform size. On the other hand, the shape of the particles turned from spherical to cubic using sodium hydroxide. The magnetic results showed that both samples attained hysteresis loop, which indicated that both samples have ferromagnetic behavior. In addition, Fe₃O₄NPs with cubic shape showed higher adsorptive behaviour towards Congo red compared to spherical Fe₃O₄NPs, which is attributed to the enhancement of their magnetic properties. The adsorption of Congo red onto cubic Fe₃O₄NPs was best described by Langmuir isotherm model, while spherical Fe₃O₄NPs followed Freundlich isotherm model.