  We present an analytic formula to extract the longitudinal structure function
in the next- to -leading order of the perturbation theory at low $x$, from the
Regge- like behavior of the gluon distribution and the structure function at
this limit. In this approach, the longitudinal structure function has the hard-
Pomeron behavior. The determined values are compared with the $H1$ data and
MRST model. All results can consistently be described within the framework of
perturbative QCD which essentially show increases as $x$ decreases.
