The nano-scale dispersed fibrils with gradient distribution in PP/PS composite fine fibers were observed by in situ formation during its melt spinning process. The morphology development of polyblends, from granule to as-spun fiber as well as drawn fiber with various PS content from 2 to 8 wt% were investigated. The morphology conversion of PS dispersed phase from ellipse to gradient nano-scale fibril along the radial direction of as-spun composite fibers took palace at 4 wt% by weight of PS component, suggesting the presence of break-up in fiber center and the limited coalescence, especially in 8 wt% PS as-spun composite fibers. This morphology diversity was attributed to the radial variation of parameters including temperature, viscosity, axial velocity and stress in spinning path and was in good agreement with the droplet deformation criteria based on the reduced capillary number. In addition, the post hot-drawing process slightly influence the size and distribution of PS phase in cross-section of composite drawn fibers, while the rheological properties of PP, PS and polyblends were found to be correlated to the morphology of PP/PS composites. (c) 2005 Elsevier Ltd. All rights reserved.