Aggregated and hyperphosphorylated Tau is one of the pathological hallmarks of Alzheimer's disease. Tau is a polyampholytic and intrinsically disordered protein (IDP). In this paper, we present for the first time experimental results on the ionic strength dependence of the radius of gyration (R_g) of human Tau 4RS and 4RL isoforms. Synchrotron X-ray scattering revealed that 4RS R_g is regulated from 65.4 to 58.5 Å and 4RL R_g is regulated from 70.9 to 57.9 Å by varying ionic strength from 0.01 to 0.592 M. The R_g of 4RL Tau is larger than 4RS at lower ionic strength. This result provides an insight into the ion-responsive nature of intrinsically disordered and polyampholytic Tau, and can be implicated to the further study of Tau-Tau and Tau-tubulin intermolecular structure in ionic environments.