In this work hierarchically porous poly(lactic acid) (PLA) with bimodal pore size distribution was derived from reactively compatibilized ternary immiscible polymer blends. We first demonstrated the generation of submicron pores from cocontinuous PLA/oxazoline functional polystyrene (PS-OX) blends. The oxazoline groups in PS-OX reacted with the carboxylic acid end groups of PLA and formed graft copolymer at the interface, which significantly reduced the domain size. Hierarchically structured "tri-continuous" morphology was further obtained in a ternary PLA/PS-OX/linear low density polyethylene (LLDPE) blend. Porous PLA was made by selectively removing the PS-OX and LLDPE phases. Static annealing was used to control the pore size. The pores formed by PS-OX (similar to 0.5-2 mm) were one order of magnitude smaller than the pores formed by LLDPE (similar to 5-20 mm). The underlying thermodynamic mechanism for formation of hierarchical morphology in PLA/PS-OX/LLDPE blend was studied, and it was found that PLA/PS/PE ternary blends demonstrated complete wetting behavior with PS located at PLA/PE interface. Our results show that compatibilization with graft/block copolymer is a versatile way to achieve hierarchical microstructures in multiphase polymer blends. (C) 2017 Elsevier Ltd. All rights reserved.