  Hydrogen recombination lines (RRLs) are one of the major diagnostics of the
physical properties of H{\sc ii} regions. We use RRL H40$\alpha$, He40$\alpha$
and 3 mm continuum emission to investigate the properties of a large sample of
resolved UC H{\sc ii} regions identified in the ATOMS survey. In total, we
identify 94 UC H{\sc ii} regions from H40$\alpha$ emission. The basic
parameters for these UC H{\sc ii} regions such as electron density, emission
measure, electron temperature, ionic abundance ratio (n$_{\rm He^+}$/n$_{\rm
H^+}$), and line width are derived. The median electron density and the median
n$_{\rm He^+}$/n$_{\rm H^+}$ ratio of these UC H{\sc ii} regions derived from
RRLs are $\sim$9000 cm$^{-3}$ and 0.11, respectively. Within UC H{\sc ii}
regions, the n$_{\rm He^+}$/n$_{\rm H^+}$ ratios derived from the intensity
ratio of the He40$\alpha$ and H40$\alpha$ lines seems to be higher in the
boundary region than in the center. The H40$\alpha$ line width is mainly
broadened by thermal motion and microturbulence. The electron temperature of
these UC H{\sc ii} regions has a median value of $\sim$6700 K, and its
dependence on galactocentric distance is weak.
