In this paper, we present the feasibility of rational function method (RFM) in order to measure the complex dielectric parameters (CDPs) of biological tissues with an open-ended coaxial probe. For this aim, the CDPs of a blood-mimicking phantom were retrieved from reflection coefficients through the RFM model from 0.5 to 6 GHz. The results were compared with commercially available Speag Dielectric Assessment Kit (DAK) software and literature. The calculated percentage dielectric property differences with the literature are 7.1% for blood. The results demonstrated that the RFM method is capable of retrieving the CDPs of biological tissues with the open-ended coaxial probes.