Doppler-broadened absorption spectrum of rubidium (87Rb and 85Rb) D2 line has been studied in the presence of a magnetic field of intermediate strength i.e. up to 5mT. Resonant non - linear magneto optical rotation is measured in the Doppler broadened spectrum of rubidium atomic vapour. Low light intensity approximation in the intermediate field region has been made to calculate the magneto optical rotation and to compare with experimental data. The enhancement of non-linear magneto optical rotation is discussed taking into account hole burning and circular birefringence in the intermediate field region. © 2007 IOP Publishing Ltd.