Fourier transform infrared and nuclear magnetic resonance results suggest that the carboxylic acid groups of poly (lactic acid) (PLA) molecules react with the hydroxyl groups of FePol (FP) molecules during the melt-blending of PLA(x)FP(y) specimens. Differential scanning calorimetry (DSC) and dynamic mechanical analysis (DMA) experiments of PLA and PLA/FP specimens suggest that only small amounts of poor PLA and/or FP crystals are present in their corresponding melt crystallized specimens. In fact, the percentage crystallinity, peak melting temperature, and onset re-crystallization temperature values of PLA/FP specimens reduce gradually as their FP contents increase. However, the glass transition temperatures of PLA molecules found by DSC and DMA reduce to a minimum value as the FP contents of PLA(x)FP(y) specimens reach 6 wt %. Further DMA and morphological analysis of PLA/FP specimens reveal that FP molecules are compatible with PLA molecules at FP contents equal to or less than 6 wt %, as no distinguished phase-separated FP droplets and tan delta transitions were found on fracture surfaces and tan delta curves of PLA/FP specimens, respectively. In contrast to PLA, the FP specimen exhibits highly deformable and tearing properties. After blending proper amounts of FP in PLA, the inherent brittle deformation and poor tearing behavior of PLA were successfully improved. Possible reasons accounting for these interesting crystallization, compatible and tearing properties of PLA/FP specimens are proposed. (C) 2010 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 48: 913-920, 2010