We show that terahertz (THz) time-domain spectroscopy (TDS) can be used to characterize the blood. The complex optical constants of blood and its constituents, such as water, plasma, and red blood cells (RBCs), were obtained in the THz frequency region. The volume percentage of RBCs in blood was extracted and compared with the conventional RBC counter results. The THz absorption constants are shown to vary linearly with the RBC concentration in both normal saline and whole blood. The excellent linearity between the THz signal and the RBC concentration was also confirmed in a polyurethane resin tube using a THz imaging method. These results demonstrate that THz-TDS imaging can facilitate the quantitative analysis of blood.