The relativistic self focusing of an intense laser beam, propagating in plasma with longitudinal static magnetic field, has been studied. The laser with Gaussian intensity distribution propagates in plasma. The extent of self focusing increases with increasing of magnetic field. Both the minimum beam width parameter and the self focusing length decrease with increase the magnetic field. The relativistic self focusing of laser beam is increased with the increase of plasma density and laser intensity, also the laser beam gets self-focused at minimum values and then diverges due to the natural diffraction by the plasma medium, self-focusing in a plasma can balance the natural diffraction and laser beam.