Abstract: An open resonator system is designed and constructed for the accurate measurement of complex permittivity at Ka band.It is solved successfully that how to obtain the accurate cavity length over a broad band,how to determinate the unique permittivity,how to complete the systematic error analysis and how to check a measurement system.The error analysis shows that the relative standard deviation of the measurement system is less than 0.17% for permittivity and 20.4% for loss tangent.Software that is developed to controls the system improves the measurement efficiency greatly.