In-situ synthesis of spinel cobalt aluminate nanoparticles using aloe vera plant extract was carried out under microwave combustion (MCM) and conventional combustion (CCM) methods. On heating, the precursors yielded nano-sized CoAl2O4 spinel, which was characterized by X-ray diffraction (XRD), Fourier-transform infrared spectrometry (FT-IR), Scanning electron microscopy (SEM), UV-Visible diffuse reflectance spectroscopy, and photoluminescence spectroscopy. The magnetic properties of the synthesized aluminates were investigated by room temperature vibrating sample magnetometry (VSM). The formation of a pure CoAl2O4 phase was confirmed by XRD and FT-IR. Both the phase composition and the crystallite size of the spinel are controlled by the plant extract. The relatively low saturation magnetization (23.96 emu/g) of CoAl2O4-CCM shows that it is weak ferromagnetic in nature and high saturation magnetization (32.13 emu/g) of CoAl2O4-MCM confirms its ferromagnetic behavior. © 2014 Elsevier Ltd and Techna Group S.r.l.