We present the analysis of single epoch long slit spectra of the three brightest images of the gravitationally lensed system J1131-1231. These spectra provide one of the clearest observational evidence for differential micro-lensing of broad emission lines (BELs) in a gravitationally lensed quasar. The micro-lensing effect enables us: (1) to confirm that the width of the emission lines is anti-correlated to the size of the emitting region; (2) to show that the bulk of Fell is emitted in the outer parts of the Broad Line Region (BLR) while another fraction of Fell is produced in a compact region; (3) to derive interesting informations on the origin of the narrow intrinsic Mg II absorption doublet observed in that system.