Cellulose acetate (CA) composite nanofibrous membranes were electrospun by introducing carboxylated cellulose nanocrystals (CCNCs) as organic nanoreinforcements which were prepared by firstly extracting the cellulose nanocrystals from microcrystal celluloses with hydrochloric acid under hydrothermal condition, and followed by oxidization with ammonium persulfate at different temperatures. The influences of the degree of carboxylation and particle size of the CCNCs on the morphology, chemical structure, thermal stability and mechanical properties of as-prepared CA composite nanofibrous membranes were investigated by scanning electron microscopy, Fourier-transform infrared spectroscopy, wide angle X-ray diffraction analysis, thermogravimetric analysis and mechanical test. Furthermore, contact angle measurement and drug release test were used to evaluate the feasibility of these nanofibrous membranes as a sustained release drug delivery system. It is found that due to the incorporation of the CCNCs, remarkable improvements on the thermal stability and mechanical properties as well as drug delivery behavior of CA nanofibrous membrane could be achieved. Especially for the CCNCs prepared at 75 A degrees C, a long-term sustained drug delivery property with a maximum drug release content of 94% within 420 h could be obtained, benefiting from the strong hydrogen bonding between CCNCs and CA matrix, high chain orientation of CA and microstructure of nanofibrous membranes. Therefore, this kind of totally cellulose-based nanofibrous membranes would possess great potential as promising materials for controlled drug delivery applications.