Various extracorporeal circulation devices require a real time thrombus detection system. This paper presents a study toward the establishment of real-time thrombus detection by measuring the electrical properties of blood. We conducted experiments on static bovine blood samples and determined the change in relative permittivity of the samples by changing the size and red blood cell (RBC) concentration of the thrombus in each sample. Results show that the relative permittivity increases linearly with increases in the number of RBCs that form the thrombus. Similarly, permittivity also increases linearly with increases in the volume of the blood that forms the thrombus. These results are consistent with the numerical simulations. However, we found the linear relationship to be dependent on the AC frequency of the applied voltage. We compare and explain this dependency on the basis of earlier studies.