The poly(lactic acid) (PLA) with 3.6% mol of benzophenone (BP) per repeat unit of PLA can be crosslinked to a gel fraction of 98.5% at UV energy of 160 J/cm2. From NMR analysis, the photocrosslinking of PLA was attributed to the recombination of primary and tertiary carbon radicals in the repeating units which were generated by the hydrogen abstraction of BP. With increasing crosslinking, melting peak disappeared and glass transition temperature elevated with loss of crystallinity, indicating that the crosslinking occurred in the crystalline region as well as in the amorphous region. The thermal stability of the crosslinked PLA also improved as shown by higher onset temperature as much as 56 degrees C and higher maximum decomposition temperature compared with the pristine PLA. The higher toughness of the crosslinked PLA film was obtained together with the improvement in tensile strength and modulus with increasing crosslinking density. In addition to photocrosslinking of amorphous region, the crosslinking of crystalline region can have significant influence in the improvement of thermal and mechanical properties. (c) 2012 Wiley Periodicals, Inc. J. Appl. Polym. Sci., 2013