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Gas transmission system operator relies on a simulation model to predict unforeseen event caused by several gas flow scenarios. Absolute roughness is required to construct the simulation model based on the Colebrook friction factor equation. It is measured by comparing operation data with steady-state results of the model or best known as tuning activities. However, finding a steady-state condition is not easy on the long transmission pipeline. The difference between supply and demand behavior generates a transient effect, hence the steady-state model is obsolete. Absolute roughness determination under transient condition has been carried out on South Sumatera to West Java (SSWJ) gas transmission pipeline. The results show that the absolute roughness during five years of observation ranging between 0.0001 to 0.0005 inch. It increases by 0.0001 inch/year but decreases by 0.0002 inch after cleaning pig activities was conducted. The results indicate that change in absolute roughness of SSWJ pipeline is determined by fouling instead of corrosion mechanism. [ABSTRACT FROM AUTHOR]