A comparison is made of the line source (LS) method and the differential line source (DLS) method of measuring thermal conductivity of particulate materials in vacuum. The DLS method requires more instrumentation in the measuring circuitry (an additional amplifier and a differentiating circuit), but since it does not require a stable temperature to initiate a test, it does not need a sample temperature control system. DLS tests can be taken as the temperature in the samples is rising from liquid nitrogen temperature to room temperature. This eliminates the practice of extrapolating thermal conductivity over this large temperature range. Also, the advantages of reduced test time, data reduction time, and small sample temperature rise enable the experimenter to take about 7–12 DLS tests in the time of 2 LS tests. Test data from the two methods agree very well. The DLS method gives slightly lower conductivity values for the small particles tested. The difference between the two methods is smaller at low temperatures than at high temperatures. The agreement in the large particle tests at room temperature is good. An error analysis shows that the LS and the DLS have total relative errors of 18 and 20%, respectively.