Composites have been widely applied in various industries and are beneficial in attaining complicated functionalities. Particularly, for dental fiber posts or orthopedic implants, the composites should have excellent mechanical properties and good imaging effects for visualization in vivo. The traditional method to improve mechanical strength and visibility by adding reinforcing fillers and radiopacifiers is complicated and has poor distributions and long production times. Hence, fabricating an integrated reinforced filler with radiopacity is of considerable economic and social significance. After ball-milling and sintering quartz fiber (QF) and bismuth trioxide (Bi2O3), a multifunctional filler (QF-Bi2SiO5) is fabricated to impart excellent flexural strengths and high X-ray imaging qualities to the composites. A composite made of epoxy resin (EP) and QF-Bi2SiO5 has a high bending strength (126.87 ± 6.78 MPa) and bending modulus (3649.31 ± 343.87 MPa), which are attributed to the tight mechanical interlock between EP and micro/nano structures of QF-Bi2SiO5. The QF-Bi2SiO5/EP composite shows good X-ray imaging quality owing to the Bi2SiO5 crystal. Furthermore, the mechanical and imaging performances of various composites with commercial fillers were compared with that of the QF-Bi2SiO5/EP composite. No filler was found that can perform both functions as well as QF-Bi2SiO5. Hence, the fabricated composites containing micro/nano structured QF-Bi2SiO5 fillers have the potential to be used in a variety of fields requiring mechanical strength and X-ray imaging capability.