The intent of this work is to determine the roughness of the inner surface of natural gas transmission pipelines for use in numerical models for pipelines flow simulation. A preliminary analysis concerning the physical properties of natural gas, including compressibility factor and dynamic viscosity is conducted with the aim of reducing the uncertainty associated with these parameters, necessary to obtain the friction factor and hence the roughness. In the work, are evaluated: seventeen models to calculate the compressibility factor, twelve models for the calculation of dynamic viscosity and six correlations for determining the friction factor in ducts. Operational data of temperature, pressure and flow rate are obtained for four different sections of the Bolivia-Brazil pipeline. The operational data are conditioned to remove transient profiles and the absolute roughness is calculated using isothermal and steady state equation. The results indicate that the roughness calculated by this method, shows dependence on gas flow through the pipeline. It was found that higher levels of flow are correlated with lower roughness values. The results also suggest a slight change in the calculated value of the roughness during the seven years that the evaluation was conducted.