Alginate ultrafine fibers are promising materials for tissue engineering applications. However, the alginate content in electrospun alginate/PEO composite fibers is limited to 85 wt%. PEO is proven to be inoperative and dispensable in cell culture experiments, because it keeps cells from attaching to the surrounding area before it is released from fibrous mats. Here, alginate ultrafine fibers are fabricated by centrifugal spinning in which alginate content is dramatically increased to 97.5 wt%. We investigate the correlation between centrifugal spinnability and alginate solution properties together with processing parameters, specifically including alginate/PEO mass ratios, PEO molecular weights, surfactant mass ratios and heater temperature. Results show that the solvent evaporation rate is a critical factor that regulates the centrifugal spinnability of aqueous alginate/PEO system. The role of PEO is to form hydrogen bonds to alginate, and thus to weaken the interactions between alginate and water. In view of the biocompatibility and biodegradability of alginate, the alginate-riched ultrafine fibers fabricated in this article may be in favor of applications in tissue engineering. POLYM. ENG. SCI., 58:1644-1651, 2018. (c) 2017 Society of Plastics Engineers