Poly (lactic acid) (PLA), made from natural resources such as starch of plants, is one of typical biodegradable thermoplastic polymers and has extensively been used in medical fields such as orthopedics, neurosurgery and oral surgery as bone fixation devices mainly due to biocompatibility and bioabsorbability (Higashi et al., 1986; Ikada et al., 1996; Middleton & Tipton, 2000; Mohanty, 2000). Its importance has led to many studies on its mechanical properties and fracture behavior which found that the mode I fracture behavior of PLA is relatively brittle in nature (Todo et al., 2002; Park et al., 2004, 2005, 2006). Therefore, blending with a ductile biodegradable and bioabsorbable polymer such as poly (e-caprolacton) (PCL) has been adopted to improve the fracture energy of brittle PLA (Broz et al., 2003; Dell’Erba et al., 2001; Chen et al., 2003; Todo et al., 2007; Tsuji & Ikada, 1996, 1998; Tsuji & Ishizuka, 2001; Tsuji et al., 2003); however, it was also found that phase separation originated by immiscibility of PLA and PCL tends to degrade the mechanical properties of PLA/PCL blends (Todo et al., 2007). It has recently been found that such phase separation can dramatically be improved by using an isocyanate group, lysine tri-isocyanate (LTI) (Takayama et al., 2006; Takayama & Todo, 2006; Harada et al., 2007, 2008), and the fracture properties of PLA/PCL/LTI are much higher than those of PLA/PCL. In this chapter, firstly the fracture behavior and micromechanism of pure PLA are summarized (Park et al., 2004, 2005, 2006; Todo et al., 2002). Effects of crystallization behavior and loading-rate on the mode I fracture behavior are discussed. Effect of unidirectional drawing on the fracture energy is also presented as one of the effective ways to improve the brittleness of PLA (Todo, 2007). Secondly, the fracture behavior of PLA/PCL blends is discussed on the basis of the relationship between the microstructure and the fracture property (Todo et al., 2007b). In the third section, improvement of microstructural morphology of PLA/PCL by using LTI is discussed (Takayama et al., 2006; Takayama & Todo, 2006; Todo & Takayama, 2007; Todo et al., 2007a). It has been found that addition of LTI effectively improves the phase morphology of PLA/PCL, resulting in dramatic improvement of fracture energy. Effects of annealing on the mechanical properties of PLA/PCL/LTI blend are discussed in the last section (Takayama et al., 2011). It has been found that a thermal annealing process can effectively improve the mechanical properties of the polymer blend, as a result of strengthened structures due to crystallization of PLA.