  We present radio observation of a millisecond pulsar PSR J0621+1002 using the
Five-hundred-meter Aperture Spherical radio Telescope (FAST). The pulsar shows
periodic pulse intensity modulations for both the first and the third pulse
components. The fluctuation spectrum of the first pulse component has one peak
of 3.0$\pm$0.1 pulse periods, while that of the third pulse component has two
diffused peaks of 3.0$\pm$0.1 and 200$\pm$1 pulse periods. The single pulse
timing analysis is carried out for this pulsar and the single pulses can be
divided into two classes based on the post-fit timing residuals. We examined
the achievable timing precision using only the pulses in one class or bright
pulses. However, the timing precision improvement is not achievable.
