  We report the first report on cyclotron line studies with the LAXPC
instrument onboard AstroSat of the High mass X-ray Binary pulsar 4U 1538-52.
During the observation of source which spanned about one day with a net
exposure of 50 ks,the source X-ray flux remained constant. Pulse profile is
double peaked in low energy range and has a single peak in high energy range,
the transition taking place around the cyclotron line energy of the source.
Cyclotron Scattering Feature (CRSF) is detected at $\sim$ 22 keV with a very
high significance in phase averaged spectrum. It is one of the highest signal
to noise ratio detection of CRSF for this source. We performed detailed pulse
phase resolved spectral analysis with 10 independent phase bins. We report the
results of pulse phase resolved spectroscopy of the continuum and CRSF
parameters. The cyclotron line parameters show pulse phase dependence over the
entire phase with a CRSF energy variation of $\sim$ 13% which is in agreement
with previous studies. We also confirm the increase in the centroid energy of
the CRSF observed between the 1996-2004 (RXTE) and the 2012 (Suzaku)
observations, reinforcing that the increase was a long-term change.
