Effects of bimodal distribution of micro- and nano-HA particles on the mechanical properties such as bending strength, modulus and J-integral at crack initiation, Jin, of HA/PLLA composites were investigated. The Bending properties were improved by bimodal distribution. It is considered that in bimodal HA/PLLA, PLLA molecular chains are constrained by existence of HA particles, resulting in the reduction of deformation of molecular chains. Jin was also effectively improved by bimodal distribution. Additional increase of dissipated energy due to dense ductile deformation of the matrix and debonding of the particle/matrix interfaces is thought to be the primary mechanism for the improvement of Jin.