The effects of blend weight ratio and polyester block length of methoxy poly(ethylene glycol)-b-poly(D,L-lactide) (MPEG-b-PDLL)/methoxy poly(ethylene glycol)-b-poly(ε-caprolactone) (MPEG-b-PCL) blends on nanoparticle characteristics and drug release behaviors were evaluated. The blend nanoparticles were prepared by nanoprecipitation method for controlled release of a poorly water-soluble model drug, indomethacin. The drug-loaded nanoparticles were nearly spherical in shape. The particle size and drug loading efficiency slightly decreased with increasing MPEG-b-PCL blend weight ratio. Two distinct thermal decomposition steps from thermogravimetric analysis suggested different blend weight ratios. Thermal transition changes from differential scanning calorimetry revealed miscible blending between MPEG-b-PDLL and MPEG-b-PCL in an amorphous phase. An in vitro drug release study demonstrated that the drug release behaviors depended upon the PDLL block length and the blend weight ratios but not on PCL block length. © 2012 Baimark and Srisuwan.