The influence of fibrinogen concentration on blood dielectric properties has been analyzed by GHz electrical impedance spectroscopy (EIS). The complex impedances of native blood and blood with various fibrinogen concentrations Z*_blood,exp were measured by a coaxial sensor in the frequency range from 1 MHz to 3 GHz. The complex permittivity of native blood and blood with various fibrinogen concentrations ε*_blood were extracted from the Z*_blood,exp by equivalent circuit model based on the transmission line theory. The reactance X_blood,native and resistance Rs_blood,native of native blood have a peak called characteristic frequency f_c at around 300 MHz. At the time t = 0 min just after fibrinogen addition, the relative blood permittivity ε_blood decreases, conductivities σ_blood increases and f_c shifts to higher frequency with increase of fibrinogen concentrations c_fib in plasma. With increment of time, from t = 0 min to t = 12 min, ε_blood decreases while σ_blood slightly decreases to time because red blood cell (RBC) aggregation reaction. By comparing the ε_blood,native of native blood and blood with various c_fib, the fibrinogen dissolved in plasma rises the blood permittivity. However, fibrinogen is unable to rise the blood permittivity unlimitedly because of RBC aggregation reaction.