The main objective of this study is to thoroughly investigate and comparing empirical and simulation results for two-phase flow during the pigging in hilly-terrain pipelines to calculate the amount of slug versus pig velocity. The high slug volume was received in slug catcher after pigging operation of sea lines related to phases 4 and 5 gas plant located in south pars gas field, south of Iran, caused liquid carry over to down stream units and total plant shut down. Beside of liquid level in slug catcher, some parameters such as liquid bottle level, and slug catcher pressure were measured during pigging time process. The results showed that during pigging process, the liquid slug volume was increased from 2100 (m3) to 4100 (m3) by decreasing the gas flow rate from 700 (ton/hr) down to 400 (ton/hr) during pigging operation. The experimental data were compared with the results of mathematical modeling of process by using SPS dynamic simulation software. The relative errors between experimental data and mathematical modeling of the process were 23.8% and 9.1% for gas flow rates of 700 (ton/hr) and 400 (ton/hr), respectively.