ISU (In-arm hydropneumatic Suspension Unit) are popularly applied to recently developed tracked vehicles because of better performance of ISU than the one of the conventional HSU (Hydraulic Suspension Unit). However, function deterioration and malfunction phenomena of the several ISUs were observed due to leakage of hydraulic fluid during operation of the tracked vehicles. The objective of this study is the investigation of fundamental leakage mechanism of hydraulic fluid for the ISU. The leakage mechanism was investigated by comparing both results of the technical data analysis and overhaul inspection of the ISU which occurred leakage of hydraulic fluid. Finally, the leakage mechanism found in the current investigation was verified by using experimental methods.