In recent years, the preparation of nanostructured architectures is an important strategy for improving gas sensing performance. These materials have an extremely high surface-to-volume ratio and high nanoporosity structures, which increases the adsorption of gases. Among these nanomaterials, functionalized graphene has generated considerable interest in sensing applications owing to its variable conductivity, which makes it available for electron transport phenomena with very high electrical mobility in the presence of oxidizing and reducing gases. Additionally, polyaniline (PANI) is easily synthesized and its molecular chain structure can be modified conveniently by copolymerization or structural derivations. Its unique electrical, electrochemical, and optical properties can also be utilized as efficient sensors for monitoring organic and inorganic compounds. Therefore, in this research, amino functionalized graphene with polyaniline as sensing active materials were deposited onto substrate surfaces of poly(methyl meth-acrylate) (PMMA) electrospun nanofibers. These nanofiber substrates offer advantages such as large surface area and high porosity, which is expected to lead to increase the sensitivity and recovery of the gas sensor. In this work, AmG/PANI/PMMA electrospun nanofibers were prepared by in situ polymerization and their efficiency as a new material for sensing CO2 gas were investigated and compared. The morphology of the samples was characterized by scanning electron microscopy (SEM) and the measurements of the gas sensing properties were tested under laboratory conditions (30-35% relative humidity, RT) using a Palm-Sens3 device. The resulting AmG/PANI/PMMA sensor showed high sensitivity toward CO2 gas at a concentration of 20 ppm, fast response and recovery times (10 and 20 s, respectively), and good stability.