Maleic anhydride (MAH) was photografted onto low-density polyethylene (LDPE) films with a grafting efficiency of about 70% in the absence of a photoinitiator. The self-initiating performance was attributed to a mechanism of abstracting hydrogen atoms from LDPE chains by excited MAH dimers. The supporting experimental results were as follows: (1) the far-UV radiation (200-300 nm) was indispensable for the graft polymerization and 2) the crosslinking reaction of LDPE inevitably accompanied the grafting of MAH. In addition, the initiation performance of MAH was further confirmed by surface photografting of acrylic acid in the presence of MAH, where MAH was used as the photoinitiator.