Rheological information is often used to determine viscoelastic fluid properties, and to model and predict fluid behavior under influence of external stress or deformation. Many industrial processes involve the dissolution of gas under high pressure, so it is important to evaluate the rheological properties of viscoelastic materials under high pressure. In this research, a high pressure rotational rheometer was developed to measure the rheological parameters of viscoelastic fluids and investigate the influence of the dissolution of gases on rheology. The rheometer utilized a piezoelectric torque transducer, which enabled transient and dynamic rheological measurements under high pressure. First, the rheometer was designed, fabricated, and calibrated using a calibration fluid. Second, the capability of the rheometer was verified using polydimethylsiloxane (PDMS), which is a typical viscoelastic fluid. Viscosity and viscoelastic properties, such as storage/loss modulus, and complex viscosity, were evaluated. Thirdly, the effects of the dissolved CO₂ on the rheological properties of PDMS were investigated. The effects of temperature and dissolved CO₂ were investigated individually at the temperature of 25, 50, 80°C and CO₂ saturation pressures of 1, 2, 3 MPa. Then, the combined effect was correlated using a generalized Arrhenius model. The proposed model expressed viscosity as a function of temperature and pressure without the need for thermodynamic and volumetric information of the fluid. The achievement of this research provides an alternative method to measure rheological properties of viscoelastic materials under high pressure and enables the prediction of the viscosity of a fluid with dissolved gas through modelling.