Spider silk and poly-L-lactic acid (PLLA) are both good bio-medical materials. PLLA/spidroin composite nano-fibers were obtained by electrospinning with the mixed solution of Ornithoctonus huwenna spider silk/1,1,1,3,3,3,-hexafluoro-2-propanol (HFIP) and PLLA/HFIP. The morphology, molecular conformations, crystallinity degree and mechanical property of the electrospun fibers were investigated by means of SEM, FT-IR, XRD, and tensile test. The results indicated that spider silk protein attenuated the fiber diameter and enhanced the mechanical property of the electrospun PLLA fibers. The diameter of electrospun PLLA fibers were micron level (>500 nm), while with the addition of spider silk protein, the diameter of the PLLA/spidroin composite fibers decreased significantly, reaching nano-scale (<500 nm), and the diameter of the composite fibers displayed a bipolar distribution of coarse fibers (>300 nm) and fine fibers (<300 nm). The FT-IR and XRD analysis showed that the blend nano-fibers had higher crystallinity, and a phase separation occurred. The mechanical property of the electrospun fiber mat was improved by adding a certain amount of spider silk protein in the PLLA/HFIP solution. When the mass fraction of the spider silk protein was 4%, the breaking strength of the electrospun PLLA/spidroin composite nano-fiber mat was 112% of pure PLLA micro-fiber mat's. Copyright (C) 2010 John Wiley & Sons, Ltd.