  The observational tests for the period of reionization of the universe are
discussed. We show that this period can be observed as {\it negative} intensity
patches of the CMB radiation with the amplitude $\delta T/T\sim 10^{-5}$ and
the angular sizes $\theta_T\sim$10 angular seconds in range of the wavelength
0.1 cm$\leq\lambda\leq$1 cm. The expected number density and frequency
dependence of the amplitude permit to recognize this effect and to discriminate
it from the noise. This method applied to the small scale variations of CMB
temperature complements well the traditional spectral approach.
  The number density and the amplitude of observed 'negative' intensity patches
depend upon the redshift of reionization that allow to estimate roughly this
redshift. The ionized bubbles formed just before the period of reionization
could also be seen as the highest peaks. The expected results are sensitive to
the Jeans scale at the period of reionization, to the small scale shape of the
primordial power spectrum and to the mass of dark matter particles.
