We have observed the micro-quasar V4641 Sgr ( SAX J1819.3-2525 ) at the time of flare during May 2002 , using the Giant Metrewave Radio Telescope at low radio frequencies ( 244 and 610 MHz ) .
This is the lowest frequency radio detection of the source .
The source showed clear signature of spectral evolution from optically thick to thin state .
We model the spectral evolution of the source to obtain some of the source parameters .
Assuming the value of magnetic field strength to be in the range of 0.1 to 1 Gauss , we obtain the initial velocity of expansion of the jet to be in the range \sim 0.45c to 0.25c .
This is consistent with the known expansion velocities of the jet with other microquasars .