The high-speed densification behavior of nanocrystalline 3 mol% yttria-stabilized tetragonal zirconia polycrystal compacts during electric current-activated/assisted sintering (ECAS) in the initial sintering stage (relative density: < 0.92) was investigated. Fully dense bulk samples with an average grain size of about 80 nm could be obtained by densification at low temperatures (T < 1473 K) under a constant external applied stress (Sigma=46.5 MPa). The densification parameters n and Q, estimated by applying a hot-pressing kinetic equation to results obtained from a quasi-constant heating rate method, instead of a conventional isothermal method, were not constant, but increased continuously with the decreasing effective external applied stress (i.e., increasing relative density). However, n and Q values that were recalculated by taking into consideration the threshold stress (Sigma(0)) were n=2 and Q=459.89 +/- 26.02 kJ/mol, respectively. These recalculated values are quite similar to those of creep deformation with an accompanying Sigma(0) in the intermediate-stress region, suggesting that the rapid densification behavior during ECAS in the initial stage of sintering is controlled by grain-boundary sliding.