Abstract: Take a 210-km-long oil pipeline flushing to convey gas as an example, the paper introduces the flush ing program, technological process, calculation methed of related parameters and pigging train structure. This pipeline section was a lenght of Puyang-Linyi oil pipeline, which was 20 year-old operated oil pipeline. To meet the needs of urban construction and environmental protection.a decision to make the oil pipeline to transmit gas was made in 1999. By means of physical-chemistry flushing method, a good flush ing result was obtained. In the end of 1999. natural gas was successfully transmitted through the flushed pipeline.