  We report the results of flux and spectral variability studies of all seven
{\it Nuclear Spectroscopic Telescope Array (NuSTAR)} observations of TeV
$\gamma-$ray emitting blazar Markarian (or Mrk) 501. We found strong evidence
of intraday variability in 3-79 keV X-ray light curves (LCs) of Mrk 501 during
four out of these seven observations. We examined spectral variability using a
model-independent hardness-ratio analysis and found a general
"harder-when-brighter" behaviour in two observations. We also investigated the
nature of 3-79 keV X-ray spectra of TeV blazar Mrk 501 and found that five out
of seven spectra are well described by the curved log-parabola models with
photon indices (at 10 keV) $\alpha \sim$ 2.12-2.32 and a curvature $\beta \sim$
0.15-0.28. The two other spectra are somewhat better represented by simple
power-law models with photon indices 2.70 and 2.75. We briefly discuss
available physical models to explain our results.
