Abstract: Al-Ti-Nb-B refiner shows excellent grain refinement effect for Al-Si alloys compared to Al-5Ti-B refiner, but the addition of Nb increases the cost of production. This paper proposes a method of preparing Al-Ti-Nb-B refiner from Nb₂O₅ as the Nb source, which not only substantially reduces the production cost, but also significantly refines the grain size. The α-Al grain size of Al-10Si alloy sharply decreases from 1727 ± 226 μm to 133 ± 10 μm and the elongation obviously increases by 178.6%. The refiner formation mechanism is revealed by thermodynamic calculations in the Al-Si-Ti-Nb-B-O system. Electron back-scattered diffraction (EBSD) analysis reveals three types of grain refiners in the whole system, which are MB₂, MAl₃ (M = Ti, Nb), and Al₂O₃. Six hitherto-unreported crystallography orientation relationships between MB₂ and Al₂O₃ with α-Al are found. Our findings provide new insights into the crystallographic orientation between grain refiners and α-Al and reveal Al-Ti-Nb-B refiners’ potential heterogeneous nucleation mechanism.