Abstarct: All oil reservoirs, including carbonate reservoirs, pressure drop and production decrease after the initial oil recovery. In carbonate reservoirs, due to being oil-wetting, huge amounts of oil still remain in the reservoir. For this reason, methods of enhance oil recovery to obtain the remaining oil in these reservoirs are of great importance. Chemical oil recovery is one of the methods of oil recovery. In this method, chemicals such as alkalines, surfactants and polymers are used. The simultaneous use of this method with nanoparticles improves oil recovery by wettability alteration of carbonate rock from oil-wet to water-wet. Therefore, in this research, the effect of alumina-baxsed nanofluids on the wettability alteration of oil-wet carbonate cores was investigated. For this work, six slices of carbonate core samples were placed in six different concentrations of nanofluids and their wettability alteration were calculated by measuring the contact angle using the sessile drop method. The results show that alumina nanoparticles have a significant effect on reducing the contact angle (toward water-wet). With the increase in the concentration of alumina nanoparticles, the change of wettability towards water-wet has increased. In the following, the effect of alumina nanoparticles in six concentrations on the viscosity of crude oil has been investigated. The results show that alumina nanoparticles have not shown a good performance in reducing the viscosity of crude oil.