  In Data Release 9 of LAMOST, we present measurements of v sin i for a total
of 121,698 stars measured using the Medium Resolution Spectrograph (MRS) and
80,108 stars using the Low Resolution Spectrograph (LRS). These values were
obtained through a chi^2 minimisation process, comparing LAMOST spectra with
corresponding grids of synthetically broadened spectra. Due to the resolution
and the spectral range of LAMOST, v sin i measurements are limited to stars
with effective temperature (Teff) ranging from 5000 K to 8500 K for MRS and
7000 K to 9000 K for LRS. The detectable v sin i for MRS is set between 27 km/s
and 350 km/s , and for LRS between 110 km/s and 350 km/s, This limitation is
because the convolved reference spectra become less informative beyond 350
km/s. The intrinsic precisions of v sin i , determined from multi-epoch
observations, is approximately 4.0 km/s for MRS and 10.0 km/s for LRS at
signal-to-noise ratio (S/N) greater than 50. Our v sin i values show
consistence with those from APOGEE17, displaying a scatter of 8.79 km/s. They
are also in agreement with measurements from the Gaia DR3 and SUN catalogs. An
observed trend in LAMOST MRS data is the decrease in v sin i with dropping
Teff, particularly transiting around 7000 K for dwarfs and 6500 K for giants,
primarily observed in stars with near-solar abundances.
