In this article, we report the preparation of a kind of novel crosslinked ultrafine fiber by electrospinning of unsaturated polyester macromonomers (UPM) and subsequent thermal crosslinking. The UPM is prepared via a two-step reaction with poly(2-methyl-1,3-propyleneadipate) diol terminated (PMPA), isophorone-diisocyanate (IPDI) and 2-hydroxyethyl methacrylate (HEMA). Poly(3-hydroxyl-butyrate-co-3-hydroxylvalerate) (PHBV) is chosen to improve the processability of the UPM. UPM/PHBV blend ultrafine fibers are successfully electrospun with a proper mass ratio of UPM to PHBV in dichloromethane solution. The fibers are thermally crosslinked after electrospinning. Measurement results indicate that the average diameter of the fibers is about 1 mu m and the crosslinked fibers have good solvent-stability and thermal-stability. This novel fiber has potential applications in filtration and protective coating. (c) 2007 Wiley Periodicals, Inc.