The pipelines located in cold region, off shore and under riverbed are exposed and unpredictable loads and these pipelines should be protected by assistant structures. As one manner of protecting river crossing pipelines, a rectangular concrete encasement is generally used in Korea. This paper describes behavior characteristics of underground pipelines encased in rectangular concrete box in terms of deformed radius derived from occurred hoop stress. The solution for contact pressure between steel pipeline and concrete encasement is derived from the equation of Lame's double walled cylinder. Every FEA model adopted in this study has same pipe diameter (762mm), internal pressure and cover depth. A variable is only the thickness of concrete encasement. This problem is formulated as a shrink-fit cylinder because of the inner steel pipe expanding by internal gas pressure. In order to get a deformed radius, the interface (contact) pressure was calculated, and the local deformed radius was determined based on obtained interface pressure and hoop stress. In this data processing, peculiar ovalized shape was developed in the section of the X65 steel pipeline covered with rectangular concrete encasement. The result in terms of the rate of local diameter change describes that shoulder (±45°, ±135°) part has the largest change rate over 120% and the smallest values occurred at a bottom (±180°). By using the relation of encasement size and deformed diameter, the results make stress design for the double layer pipeline be more precise and effective.