In this study, electrospinning conditions for ultrafine cellulose fibers was systematically studied and poly(butylene succinate) biocomposites reinforced by the ultrafine cellulose fibers (cellulose/PBS biocomposite) were fabricated. The ultrafine cellulose fibers were electrospun from cellulose (DP = 700) solutions in N-methylmorpholine-N-oxide hydrate (85/15 w/w) at 100 degrees C. The optimal electrospinning concentration of the cellulose solutions was determined to be 7 wt % and the average diameter of the resulting cellulose fibers was 560 rim. The cellulose I structure of the native cellulose was converted to the cellulose II structure after electrospinning. The ultrafine cellulose fibers showed a reinforcing effect in the cellulose/PBS biocomposite, suggesting that they have potential applications as reinforcement fibers for biocomposites. (c) 2007 Wiley Periodicals, Inc.