The insulation system in a transformer degrades over time and it needs periodic monitoring for uninterrupted operation of the power system network. Moisture content (MC), acid number, and dissolved gases are the important parameters that establish the aging and degradation status of the transformer liquid insulation. This work focuses on using high-frequency signals nonintrusively for estimating the degradation of the transformer liquid insulation by considering the MC and acid content. A novel nonintrusive and nondestructive testing method using an S-band pyramidal horn antenna is introduced in this work to estimate the degradation of the liquid insulation with respect to aging duration. The scattering parameter S-11 of the liquid insulation is first characterized and measured in an anechoic chamber to avoid electromagnetic interference (EMI). These tests are performed initially on laboratory-aged mineral oil (MO) samples and a prediction curve of vertical bar S-11 vertical bar as a function of moisture and acid number is obtained. vertical bar S-11 vertical bar of the in-service transformer oil samples is then compared with the prediction curve and the degradation status is predicted. It is found that the chosen frequency band is able to differentiate the aging status of MO accurately. This technique can be used for establishing the degradation status of any type of liquid insulation. A miniature version of the proposed technique can be used for portable and faster condition assessment of insulating oils.