  We detected a giant X-ray flare from the RS-CVn type binary star UX Ari using
MAXI on 2020 August 17 and started a series of NICER observations 89 minutes
later. For a week, the entire duration of the flare was covered with 32
snapshot observations including the rising phase. The X-ray luminosity reached
2$\times$10$^{33}$ erg s$^{-1}$ and the entire energy release was $\sim
10^{38}$ erg in the 0.5--8.0~keV band. X-ray spectra characterized by continuum
emission with lines of Fe XXV He$\alpha$ and Fe XXVI Ly$\alpha$ were obtained.
We found that the temperature peaks before that of the flux, which suggests
that the period of plasma formation in the magnetic flare loop was captured.
Using the continuum information (temperature, flux, and their delay time), we
estimated the flare loop size to be $\sim 3 \times 10^{11}$ cm and the peak
electron density to be $\sim 4\times10^{10}$ cm$^{-3}$. Furthermore, using the
line ratio of Fe XXV and Fe XXVI, we investigated any potential indications of
deviation from collisional ionization equilibrium (CIE). The X-ray spectra were
consistent with CIE plasma throughout the flare, but the possibility of an
ionizing plasma away from CIE was not rejected in the flux rising phase.
