The size dependence of optical and, thermophysical properties of Fe3O4 nanoparticle (NPs) has been investigated. Three samples of Fe3O4 NPs with different sizes were prepared by the Co-precipitation method and labeled a, b and c. The UV- vis spectra shifted toward the lower energy with increasing the particles size. The energy bandgaps of the synthesized Fe3O4 NPs are 1.84 ev,1,73 ev, and 1,47 ev for a, b and c respectively. The sizes of the nanoparticles were estimated using the effective mass approximation (EMA) model, and are 10.7 nm, 20.56 nm, and 30.43 nm. These values are comparable to those found by transmission electron microscope (TEM) and X-ray diffraction (XRD). The Photoacoustic (PA) technique was used to research Fe3O4 NPs for thermal diffusivity (α), thermal effusivity (e), and thermal conductivity (k). The determined values of α, e, and k increase as the size of nanoparticles rises and decreases from the bulk by about 33%.