A passive equalizer with negative impedance to increase a gain includes a first RC loop, a second RC loop, a cascade RL circuit and a cross-coupled inverter unit. Each of the first and the second RC loops includes a first resistor, a second resistor connected in series to the first resistor at a node to thereby form a resistor series, and a capacitor connected in parallel to the resistor series. The cascade RL circuit is connected between the RC loops and includes a fifth resistor, a sixth resistor and an inductor connected between the fifth resistor and the sixth resistor. The cross-coupled inverter unit is connected in parallel to the RL circuit and connected between the RC loops for using the feature of negative impedance to obtain an excellent high-frequency gain.