  The peak effect (PE) has been observed in a twinned crystal of
YBa$_2$Cu$_3$O$_{7-\delta}$ for H$\parallel$c in the low field range, close to
the zero field superconducting transition temperature (T$_c$(0)) . A sharp
depinning transition succeeds the peak temperature T$_p$ of the PE. The PE
phenomenon broadens and its internal structure smoothens out as the field is
increased or decreased beyond the interval between 250 Oe and 1000 Oe.
Moreover, the PE could not be observed above 10 kOe and below 20 Oe. The locus
of the T$_p$(H) values shows a reentrant characteristic with a nose like
feature located at T$_p$(H)/T$_c$(0)$\approx$0.99 and H$\approx$100 Oe (where
the FLL constant a$_0$$\approx$penetration depth $\lambda$). The upper part of
the PE curve (0.5 kOe$<$H$<$10 kOe) can be fitted to a melting scenario with
the Lindemann number c$_L$$\approx$0.25. The vortex phase diagram near T$_c$(0)
determined from the characteristic features of the PE in
YBa$_2$Cu$_3$O$_{7-\delta}$(H$\parallel$c) bears close resemblance to that in
the 2H-NbSe$_2$ system, in which a reentrant PE had been observed earlier.
