In　this　paper,　a　blind　narrowband　interference　(NBI)　mitigation　technique　using　the　combination　of　a　square　law　(SL)　device　and　a　band　pass　filter　(BPF)　is　proposed　for　energy　detection　(ED)　based　ultra-wideband　(UWB)　receivers.　Both　theoretical　analysis　and　computer　simulations　show　that　without　requiring　any　prior　knowledge　about　the　NBI,　the　SL　technique　can　considerably　improve　signal-to-interference　ratio　of　the　received　UWB　pulses　and　hence　allows　ED-based　UWB　receivers　achieve　much　better　bit　error　rate.　Furthermore,　the　proposed　SL　technique　has　the　same　NBI　mitigation　performance　as　the　Teager-Kaiser　operator　(TKO)　technique.　Unlike　TKO,　there　are　many　commercial　off-the-shelf　SL　devices　are　available　for　IR-UWB　applications.　Thus,　the　proposed　SL　technique　is　a　simple,　implementable,　and　highly　effective　blind　NBI　mitigation　technique　for　ED-based　IR-UWB　receivers.