  A good observation of preflare activities is important for us to understand
the origin and triggering mechanism of solar flares, and to predict the
occurrence of solar flares. This work presents the characteristics of microwave
spectral fine structures as preflare activities of four solar flares observed
by Ond\v{r}ejov radio spectrograph in the frequency range of 0.8--2.0 GHz. We
found that these microwave bursts which occurred 1--4 minutes before the onset
of flares have spectral fine structures with relatively weak intensities and
very short timescales. They include microwave quasi-periodic pulsations (QPP)
with very short period of 0.1-0.3 s and dot bursts with millisecond timescales
and narrow frequency bandwidths. Accompanying these microwave bursts, there are
filament motions, plasma ejection or loop brightening on the EUV imaging
observations and non-thermal hard X-ray emission enhancements observed by
RHESSI. These facts may reveal certain independent non-thermal energy releasing
processes and particle acceleration before the onset of solar flares. They may
be conducive to understand the nature of solar flares and predict their
occurrence.
