The implementation of subsea separation and liquid boosting are becoming a common development scheme for the exploration of deep water fields. Subsea separation system is an attractive and economic solution to develop subsea offshore fields producing fluid without hydrate or wax. Subsea separation system should be reliable to ensure successful operation in a wide range of multiphase flow regime without need for developments. A subsea separator can avoid or simplifying costly surface platforms of floating vessels as well as being an efficient tool to enhance hydrocarbon production. In this study, multiphase flow characteristics inside in-line type subsea separator is investigated for the design of subsea separation system.