This work introduces an innovative technique for non-destructive evaluation of two-dimensional electron gas (2DEG) quality on GaN-based HEMT wafer by mercury probes, which has huge utility value in manufacturing. As demonstrations, the carrier density dependent sheet resistance and the mobility of 2DEG are extracted with accuracy by measuring the frequency and voltage dependence of the heterostructure capacitance with two concentric-circle Schottky contacts. The principle of this technique is based on strict analytical derivation of the frequency dependence of the capacitance, in form of Bessel function, by modeling a distributed network containing differential capacitances and resistances and solving corresponding transmission line functions. We utilize a physics phenomenon, high frequency alternating current signal decays in propagation induced by the resistance of 2DEG, to develop a technique for 2DEG resistance and mobility characterization.