INTRODUCTIONInorganic nanostructures materials with fully-determined morphologies having excellent properties and probable applications have received major attention in recent years. The nanoparticles of metal oxide are prominent in inorganic material research for developing different practical applications and these nanoparticles have individual chemical properties [1][2][3] depends on their structure, composition, shape and size [4][5][6]. Copper oxide nanoparticles are the most studied materials between all the metal oxides because of interesting properties as a p-type semiconductor with the possibility of a large difference in energy band gap. Copper oxide are used in many different applications such as nanofluid [7] [19]. Of all the above synthesis procedure, it appears to be very difficult to get a pure crystal of copper oxide material without adding many stabilizing agents. Actually, as it was adduced [20], the elaboration of copper oxide nanoparticles is usually accompanied by the presence of side product such Cu2O or Cu(OH)2. For this, the defy taken up in this paper is to fabricate pure copper oxide nanoparticles, in the nano range, using novel and the low-cost photolysis method without adding any stabilizing agent. This Copper oxide nanoparticles with different size were successfully fabricated by the photolysis method through the irradiation of copper oxalate complex with different solvents. The effect of the type of solvent on the size of nanoparticles was investigated. The structure and size of nanoparticles were determined using XRD and TEM, while the spectral properties of copper oxide nanoparticles investigated using FTIR and UV-visible. XRD diffraction studies obtained pure monoclinic structure of copper oxide without secondary phase and the size of particles (8.4 to 11.4 nm) depends strongly on the dielectric constant of solvents and the smallest particles of copper oxide were showed when using the ethanol as solvent. A blue shift in the essential gap energy (from 4.21 to 4.58 eV) due to the quantum confinement effect, is obtained in the spectra analysis when the particles size decreases. Effect of Solvents on Size of Copper Oxide Nanoparticles Fabricated using Photolysis Method Keywords: Copper oxide nanoparticles, Photolysis, Irradiation, Quantum confinement.process contained irradiation the complex or salt of copper to transform it from the oxidation state (+2) to oxidation state (+1) using an ice bath to avoid the high temperature then, the product burned at 300 °C. Furthermore, as it can be abstracted from this work when the fabricate parameters are well optimized, the photolysis method can produce a pure product. In this framework, the paper aim to study the effect of the solvent nature on the size of particles of the as-prepared copper oxide nanoparticles. EXPERIMENTALCopper oxide nanoparticles were prepared using potassium oxalate and copper sulphate (Sigma Aldrich, 99 %) as precursors.Preparation of complex and copper oxide nanoparticles: In two separate beakers, 3.1 g of potassium ox...