Abstract 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 χ 2 minimization 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 an effective temperature ( T eff ) ranging from 5000 to 8500 K for MRS and 7000 to 9000 K for LRS. The detectable v sin i for MRS is set between 27 and 350 km s −1 , and for LRS between 110 and 350 km s −1 . This limitation is because the convolved reference spectra become less informative beyond 350 km s −1 . The intrinsic precision of v sin i , determined from multiepoch observations, is approximately ∼4.0 km s −1 for MRS and ∼10.0 km s −1 for LRS at a signal-to-noise ratio greater than 50. Our v sin i values show consistency with those from APOGEE17, displaying a scatter of 8.79 km s −1 . They are also in agreement with measurements from the Gaia DR3 and Sun et al. catalogs. An observed trend in LAMOST MRS data is the decrease in v sin i with a drop in T eff , particularly transiting around 7000 K for dwarfs and 6500 K for giants, primarily observed in stars with near-solar abundances.