In this work, calcium carbonate nanoparticles-polyethylene oxide nanofibers as organic-inorganic hybrid were prepared via in-situ electrospinning. Thus, electrospinning of polyethylene oxide solution saturated with calcium hydroxide was carried out in gaseous carbon dioxide atmosphere. Transmission electron microscopy (TEM) showed that calcium carbonate (CaCO(3)) nanoparticles were formed on the produced nanofibers of 200-300 nm in diameter. The existence of the formed CaCO(3) was also proved by thermogravimetric analysis (TGA) via loss of gaseous CO(2) related to the decomposition of CaCO(3) at about 500-840 degrees C. X-ray diffraction (XRD) analysis of the nanofibers; showed that the formed CaCO(3) nanoparticles have vaterite morphology. DSC analysis was used to determine melting point and to calculate the crystallinity of the produced hybrid nanofibers. The TEM, TGA, XRD and DSC analyses results of the obtained nanofibers were compared with those of the nanofibers produced in electrospinning of pure polyethylene oxide solution and polyethylene oxide solution having calcium hydroxide, both in air.