  We present the results from a systematic study of temporal variation of
stellar activity in young late-type stars. We used multi-epoch LAMOST
low-resolution spectra of over 300 member candidates in three young open
clusters: Pleiades, Praesepe, and Hyades. The spectral measurements of TiO band
strength near 7050 {\AA} (TiO2) and equivalent width of H{\alpha} line (EW
H{\alpha} ) are used as the tracers of cool-spot coverage and chromospheric
emission strength, respectively. The analysis of time-variation patterns of
these two tracers suggested there exist detectable variabilities in TiO2 and EW
H{\alpha} , and their time-scales are in the wide range from days to years.
Results showed that more active stars, younger and fast rotators, tend to have
larger activity variations. There is a tendency of anti-correlation between
temporal variations in TiO2 and EW H{\alpha} . Also, appreciable
anti-correlation in the rotational phase between H{\alpha} emission and K2
brightness is detected in some M dwarfs, indicating spatial co-location of the
plages with cool starspots, however, cool stars do not always show such
co-location features. Furthermore, spot coverage and H{\alpha} emission were
evident at all rotational phases of several M dwarfs, indicating a basal level
of activity, perhaps due to many small and randomly located active regions in
the atmosphere.
