In gravity-driven oil/water separation, porous membranes with both micro-and nano-pores, corresponding to high flux, are eagerly desired. The fabrication of them remains as a great challenge. In this work, hierarchically porous Poly(L-lactide) (PLLA) fabrics were fabricated by electrospinning PLLA/PEO (Poly ethylene-oxide) blend solution and the subsequent removal of PEO by means of water etching. Micro-pores among electrospun fibers and nano-pores in fibers resulted from the bicontinuous structures due to the segregation of PLLA during the crystallization of PEO account for the excellent gravity-driven separation performance. On one hand, hierarchical roughness owing to the existence of pores in two scales produces superhydrophobic and superoleophilic properties of the fabrics, which is the reason for the enhanced driving force for the oil/water separation; on the other hand, the nano-pores provide extra channels for the permeation of filtrate, which contributes 20% improvement of the flux. Our result opens a new avenue for fabricating hierarchically porous separation membranes for ultrafast oil/water separation.