A substantial amount of nanosized hydroxyapatite(HAp) was tried to incorporate into the electrospun polyhydroxybutyrate(PHB), and electrospinning parameters were optimized to fabricate defect-free PHB/HAp fibers with the smallest possible diameter. According to the results, while the needle inner diameter could not be an effective factor for controlling the fiber morphology, the fiber diameter was observed to increase with increasing the applied voltage and solution concentration. More importantly, it was found that nanoparticles could be successfully encapsulated and distributed inside the ultrafine fibers fabricated under relatively optimum conditions. In vitro cell assays demonstrated that while preosteoblasts had high cell viability and cell spreading on the fibrous nanohybrids, cell metabolic activity can also increase with increasing incubation time. Accordingly, encapsulation of HAp nanoparticles within the PHB in the fibrous form may be promising for bone regeneration applications. A substantial amount of nanosized hydroxyapatite (HAp) was tried to incorporate into the electrospun polyhydroxybutyrate (PHB), and electrospinning parameters were optimized to fabricate defect-free PHB / HAp fibers with the smallest possible diameter. According to the results, while the needle inner diameter could not be an effective factor for controlling the fiber morphology, the fiber diameter was observed to increase with increasing the applied voltage and solution concentration. More importantly, it was found that the nanoparticles could be successfully encapsulated and distributed inside the ultrafine fibers fabricated relatively relatively Optimal conditions. In vitro cell assays of while preosteoblasts had high cell viability and cell spreading on the fibrous nanohybrids. for bone regeneration applicati ons.