  Since the cosmic peculiar velocity field depends on small wave-number modes
strongly, we cannot probe its universal properties unless we observe a
sufficiently large region. We calculate the expected deviation (sample
variance) of the peculiar velocity dispersion from its universal value in the
case observed volume is finite. Using linear theory we show that the sample
variance remains as large as $\sim 10%$, even if the observed region is as deep
as $100h^{-1}Mpc$ and that it seriously affects the estimation of cosmological
parameters from the peculiar velocity field.
