The dielectric constant of liquid and powder materials in the millimeter wave band plays a significant role in many fields such as agriculture, food processing and biomedicine. To test the dielectric constant accurately, a test system is built with a Vector Network Analyzer (VNA), a corrugated transmission line and a special container made of polytetrafluoroethylene (PTFE). In the test, the Gated Reflection Line (GRL) method first performs calibration to both sides of the container inner wall to eliminate the multiple reflection error between the container and the material to be tested. Then the liquid or powder material to be tested is put into the container, the VNA obtains the S-parameters, and the free-space algorithm calculates the dielectric constant. Test results show that testing solid PTFE as a standard sample in the frequency band of 75GHz～110GHz verifies the feasibility of the test system. When testing a certain type of silicone oil and quartz powder, the maximum error of the test results deviating from the average value is only 5%. This indicates that the test method has high accuracy and can be used for the test of dielectric constant of liquid and powder materials in the millimeter wave band.