In this study, we investigated the phase separation phenomenon of silk fibroin/poly (-caprolactone) electrospinning solution to improve the performance of silk fibroin/poly (-caprolactone) electrospun nanofibers. It showed that phase separation does occur in just a few hours in the silk fibroin/poly (-caprolactone)/formic acid mixture solution. Acetic acid, small molecule nonsolvent for silk fibroin, was first introduced to silk fibroin/poly (-caprolactone)/formic acid solution, a homogeneous solution without separation for over several days was achieved after mixing for 5h. The morphology and composition of the silk fibroin/poly (-caprolactone) and acetic acid-modified silk fibroin/poly (-caprolactone) fibrous scaffolds were examined by scanning electron microscopy, Fourier transform infrared spectroscopy and thermal gravimetric analyzer. Attachment and proliferation of mouse osteoblast MC3T3-E1 cells were tested by scanning electron microscopy and cytotoxity assay. The results indicated that the phase separation of silk fibroin/poly (-caprolactone) solution might led to inhomogeneous morphology and composition of the composite scaffolds, and the inhomogeneity of the silk fibroin/poly (-caprolactone) scaffolds with formic acid as solvent had a remarkable difference on cell adhesion and proliferation. In contrast, there was no significant difference among the silk fibroin/poly (-caprolactone) scaffolds with formic acid/acetic acid as solvent because of their good consistency in fiber morphology and composition. These obtained silk fibroin/poly (-caprolactone) nanofibers had small average diameter of 190 +/- 40nm. The obtained results proved that this study provided a facile and effective approach to achieve compositionally homogeneous silk fibroin/poly (-caprolactone) scaffolds with formic acid as solvent for effective applications.