The viscosity of ionic liquids (ILs) plays an important role in the chemical industry, material science and environmental science. This work presents a temperature- and pressure-dependent (η-T-P) model based on the norm indices for describing the viscosity of ILs. While the parameters for the temperature and pressure dependence of the viscosity have been generally set as constants in previous work, they are adjusted based on the molecular structures in the η-T-P model. Because ILs consist of only cations and anions, the norm indices are derived from cations and anions, respectively. However, the close connection between the interaction of the cations and anions and IL viscosity cannot be neglected; therefore, another set of norms is further deduced to represent the cation-anion interaction. The η-T-P model is developed using 349 ILs (3228 data points) in the temperature range of 253.15-573.00 K and the pressure range of 0.06 to 300.00 MPa. Consequently, a η-T-P model is derived with convincing results, as evaluated by the following statistical parameters: the squared correlation coefficient (R2), Fisher significance (F) and the overall average absolute relative deviation (AARD), which are 0.964, 1809 and 4.62%, respectively.