  Radial velocity is one of key measurements in understanding the fundamental
properties of stars, stellar clusters and the Galaxy. A plate of stars in the
Kepler field were observed in May of 2018 with the medium-resolution
spectrographs of LAMOST, aiming to test the performance of this new system
which is the upgraded equipment of LAMOST after the first five-year regular
survey.We present our analysis on the radial velocity measurements (RVs)
derived from these data. The results show that slight and significant
systematic errors exist among the RVs obtained from the spectra collected by
different spectrographs and exposures, respectively. After correcting the
systematic errors with different techniques, the precision of RVs reaches ~1.3,
~1.0, ~0.5 and ~0.3 km/s at S/Nr = 10, 20, 50, and 100, respectively. Comparing
with the RVs of the standard stars of the APOGEE survey, our RVs are calibrated
with a zero-point shift of ~7 km/s. The results indicate that the LAMOST
medium-resolution spectroscopic system may provide RVs in a reasonable accuracy
and precision for the selected targets.
