Polythioether is an elastomer with proper thermal resistance and elongation at break, which can be used in various high-temperature and high-pressure sealing applications. This research is aimed to determine the suitable thiol/vinyl ratio and the branching agent content to synthesize a novel polythioether and the polythioether/ multiwall carbon nanotube (MWCNT) composite using two monomers, including dimercapto dioxaoctane (DMDO) and triethylene glycol divinylether. For each synthesis process, the gel permeation chromatography (GPC) was used to measure the molecular weight and the polydispersity index of the polymer chains. Fourier transforms infrared spectroscopy and Nuclear Magnetic Resonance were used to study the bond formation, while the glass transition of the polymer was evaluated by differential scanning calorimetry. Tensile and lap adhesion tests were also used to assess the mechanical properties of the samples. Field emission scanning electron microscopy was utilized to observe the dispersion of nanoparticles in polythioether/ MWCNT nanocomposite. GPC results showed that the thiol/vinyl ratio of 0.5 and branching agent molar ratio of 0.2 (relative to vinyl monomer) led to the most suitable average molecular weight (2022 g/mol) which had the highest mechanical properties.The nanocomposite containing 1 wt% MWCNT, showed the highest tensile strength and elongation at break up to 2.12 MPa and 303%, respectively. A considerable increase of 30 C is obtained in maximum degradation temperature of the produced nanocomposite compared to neat polythioether.