  Concepts like peculiar velocity, gravitational force, and power spectrum and
their interrelationships are of utmost importance in the theories of structure
formation. The observational implementation of these concepts is usually based
on the Newtonian hydrodynamic equations, but used up to scales where general
relativistic effects come in. Using a perturbation of FRW metric in harmonic
gauge, we show that the relativistic effects reduce to light cone effects
including the expansion of the universe. Within the Newtonian gravitation, the
linear perturbation theory of large scale structure formation predicts the
peculiar velocity field to be directly proportional to gravitational force due
to the matter distribution. The corresponding relation between peculiar
velocity field and density contrast has been given by Peebles. Using the
general relativistic perturbation we have developed, this familiar relation is
modified by doing the calculation on the light cone in contrast to the usual
procedure of taking a space-like slice defined at a definite time. The velocity
and density--spectrum are compared to the familiar Newtonian expressions. In
particular, the relativistic $\beta$--value obtained is reduced and leads to an
increased bias factor or a decreased expected amount of the dark matter in a
cluster.
