Due to the poisoning and death problems because of gas leaks, the demand for highly sensitive and highly selective gas sensors is increasing day by day. The performance of gas sensors is directly proportional to their sensitivity. As sensitivity is one of the main parameters of the gas sensors, this parameter should be improved. The sensitivity of a gas sensor is measured with the changing electrical signal of the gas-sensitive element of the sensor reaction to a gas molecule. Therefore, in this research work, six gas-sensitive elements based on multi-walled carbon nanotubes and different polymer composites were prepared and investigated. The main goal of this paper is to investigate the sensitivity of prepared elements based on MW-CNTs and polymers under CO gas. However, other main parameters like selectivity, reaction time, and recovery time, stability, limit of detection are also should be improved. In this research work, polyurethane(PU)/MW-CNT, silicone/MW-CNT, epoxy/MW-CNT, Polyvinyl acetate(PVAc)/MW-CNT,Polyvinylalcohol(PVAl)/MWCNT,Polystyrene(PS)/MW-CNTnanocomposites were prepared and sensitivity was measured in air and under CO gas. The results showed that under CO gas, only the PVAc /MW-CNT composite shows high sensitivity to CO gas. Thus, in air, the resistivity of PVAc/MW-CNT composite is 1,371 * 106 and under CO gas is 9,92 * 106 Ω which shows the change of resistivity nearly seven times. Other composites do not change resistivity under CO gas. However, only PU/MW-CNT and PS/MW-CNT also show very low sensitivity to this gas. It means that only PVAc/MW-CNT composite is suitable as an active sensitive element for the preparation of highly sensitive gas sensors.