In this work, thermal decomposition of the [Co(NH 3 ) 5 (H 2 O)](NO 3 ) 3 precursor complex was investigated under solid state conditions. Thermal analysis (TG/DTA) showed that the complex was easily decomposed into the Co 3 O 4 nanoparticles at low temperature (175 °C) without using any expensive and toxic solvent or a complicated equipment. The obtained product was identified by X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FT-IR), Raman spectroscopy, scanning electron microscopy (SEM), transmission electron microscopy (TEM) and energy-dispersive X-ray spectroscopy (EDX). Optical and magnetic properties of the products were studied by UV-visible spectroscopy and a vibrating sample magnetometer (VSM), respectively. FT-IR, XRD and EDX analyses confirmed the formation of highly pure spinel-type Co 3 O 4 phase with cubic structure. SEM and TEM images showed that the Co 3 O 4 nanoparticles have a sphere-like morphology with an average size of 17.5 nm. The optical absorption spectrum of the Co 3 O 4 nanoparticles showed two band gaps of 2.20 and 3.45 eV, which in turn confirmed the semiconducting properties. The magnetic measurement showed a weak ferromagnetic order at room temperature. Photocatalytic degradation of methylene blue (MB) demonstrated that the as-prepared Co 3 O 4 nanoparticles have good photocatalytic activity under visible-light irradiation.