Bio-based polyurethane (BPU) has been developed using a castor oil/poly (ƒÕ-caprolactone) (PCL) hybrid polyol, and hydrophobic BPU nanofibers containing modified silica were successfully fabricated using an electrospinning process. The successful modification of the silica nanoparticles and the synthesis of BPU composites were confirmed by Fourier transform infrared spectroscopy data. The rheological properties of the BPU solutions and BPU/m-silica suspensions were investigated to characterize any structural changes induced by incorporation of the modified silica and to control the electrospinning parameters. The rheological analysis revealed that a network structure existed between the BPU and modified silica, which led to a remarkable improvement in the mechanical properties and thermal stability. A morphological change in the nanofibers on incorporation of the modified silica nanoparticles was also observed: the average fiber diameter of the hybrid nanofibers decreased with increasing modified silica content. Furthermore, the modified silica nanoparticles resulted in a change in the effective surface wettability of the BPU nanofibers resulting in a change from hydrophilic to hydrophobic behavior. The present BPU/modified silica nanofiber, which has improved rheological properties, hydrophobic surface, mechanical properties, and thermal stability, may be a potential candidate to replace petroleum based polyurethane membrane, in the field of biofilters, eco-friendly textiles, and biomedical engineering. This work was supported by the National Research Foundation of Korea (NRF) Grant Funded by the Korean Government (MSIP) (Project Nos. 2014049867 and 2013-022313), the Ministry of Trade, Industry & Energy (MOTIE) and Korea Institute for Advancement of Technology (KIAT) (Project No. N0000993). Keywords: Bio-based polyurethane; Silica nanoparticle; Electrospinning; Hydrophobicity.