Various studies on specific absorption rates (SARs) have been carried out with liquid phantoms representing human body tissues in the field of electromagnetic compatibility (EMC) research. As a faithful representation of human body tissue is needed to establish an accurate SAR for measurement, knowledge regarding the precise measurement of sample materials with high permittivity and high loss is very important. In this study, the complex permittivity of a liquid phantom material was measured using the open-ended cut-off circular waveguide reflection method. The effectiveness of the proposed approach for measuring a liquid phantom with high permittivity and high loss was also confirmed by comparing the measurement results with those obtained using the TM010 circular cavity resonator method.