Together with the developing nanotechnology, nanofluids and nanoparticles are used as working fluid in energy applications. It is foreseen that nanoparticles have high heat conduction coefficient and it will increase system performance by using as a working fluid in energy systems. Many studies in the literature show that nanofluids increase the heal transfer rate by improving heat transfer. In this study a performance analysis of an absorption cooling system using solar energy was performed as numerically. LiBr-Al2O3-water nanofluid has been used in the cooling system as working fluid. The thermodynamic values and calculations used in the analyses were performed with Engineering Equation Solver program. Heat load necessary for the generator is provided with a flat plate solar collector. For different operation condition, the variation of COP values was determined depend on Al2O3 water nanoparticle concentration ratio. When the Al2O3-water nanoparticle concentrations are changed as 0%, 0.5% and 0.1%, it was determined that the COP values increased. Nanoparticles added to the refrigerant at certain concentration values affects the COP values positively of cooling systems. Maximum COP value is 0.86 for 85 degrees C generator temperature and 0.1% Al2O3 water nanoparticle concentration. The lowest COP value was obtained for the 75 degrees C generator temperature. When the Al2O3-water nanoparticle concentration was increased together with the generator temperature, COP values also increased. When the nanoparticle concentration of the working fluid increases, the viscosity of the nanofluid can be increases. Due to, increased viscosity increases the pressure drop in the flow channel and the pump power required for the flow. Thus, minimum viscosity with maximum thermal conductivity optimisation in applications is very important.