An　all-fiber　magnetic-field　sensor　combining　a　multicore　fiber　taper　and　magnetic　fluid　is　proposed　and　experimentally　demonstrated.　The　magnetic-field　sensing　mechanism　is　investigated　theoretically　and　experimentally.　Experimental　results　demonstrate　that　the　dip-wavelength　of　the　proposed　sensor　regularly　varies　with　the　changes　of　the　applied　magnetic　field.　The　measuring　sensitivity　is　achieved　to　be　~　34　pm/Gs　in　the　magnetic　induction　intensity　range　of　50-300　Gs.　Moreover,　the　sensor　responding　to　the　50-Hz　alternating　magnetic　field　is　also　experimentally　investigated,　and　a　minimal　detectable　magnetic　induction　intensity　of　$~　$23.7　Gs　is　successfully　achieved.　The　proposed　magnetic　field　sensor　is　attractive　due　to　its　compact　size,　low　cost,　and　immunity　to　electromagnetic　interference.　IEEE