In this study, a new method of preparing porous ultra-fine fibers via photocrosslinking was developed. Ultra-fine unsaturated polyester macromonomor (UPM)/poly(3-hydroxybutyrate-co-3-hydroxvalerate) (UPM/PHBV) blend fibers were electrospun and then the UPM was photocrosslinked by UV irradiation. Different ratios between UPM, PHBV and solvent were tested and the relationship between weight percentage of solutions and diameter of fibers was discussed. Through the test of T. and T., we found that UPM and PHBV were immiscible and the phase separation proceeded during the electrospinning. The photocrosslinking time was controlled strictly and the best reaction time can't exceed more thin 10 min. After photocrosslinking of UPM, PHBV was extracted from the blend fibers with chloroform. The morphology of the fiber was observed through SEM and fibers were not collapsed during the extracted processing. (C) 2009 Wiley Periodicals, Inc. J Appl Polym Sci 112: 2247-2254, 2009