A STUDY IS PRESENTED OF THE PERFORMANCES OF SEALING ELEMENTS IN THE OIL CYLINDER OF A HYDRAULIC SYSTEM IN A LOW-PRESSURE SYSTEM (THE OIL PRESSURE IS NOT MORE THAN 400 PSI). MUCH OF THE PAPER DISCUSSES THE ANALYSIS OF THE FRICTION OF THE SEALING SET, AND A FORMULA OF THE FRICTION FORCE FOR 0-RING AND RECTANGULAR-RING IS DEVELOPED. THIS PAPER SHOWS THE STRESS RELAXATION AND THERMAL EFFECT IN THE SEAL, WHICH ARE MAIN INFLUENTIAL FACTORS FOR THE SEALING PERFORMANCE. IT ANALYZES THE AMOUNT OF SQUEEZE, AND HAS A RECOMMENDED RANGE OF SQUEEZE VALUES. THE SETUP OF THE CYLINDER IS DISCUSSED. IT IS EXPECTED THAT THE PRINCIPLE OF THIS PAPER ALSO IS USEFUL TO OIL SYSTEMS OF HIGHER PRESSURES.