The method to deeply undercool alloy melts far below the liquidus temperature by eliminating heterogeneous nucleation sites inside is frequently used in studying non-equilibrium solidification behavior, preparing quasi-crystal, amorphous alloy and other metastable materials. Previous work on the solidification of Co-(18.5~20.7)%B (atomic fraction) alloys indicated that metastable Co₂₃B₆ phase instead of stable Co₃B phase was formed as the primary phase from the melts undercooled by larger than 60 K. To know whether Co₂₃B₆ phase can still primarily form from the deeply undercooled melt of Co₇₅B₂₅, the nominal composition of Co₃B phase, the Co₇₅B₂₅ alloy melt was undercooled to different degrees using the glass fluxing technique, and the solidification path was identified by analyzing the microstructures and cooling curves of the samples. There was nothing other than α-Co and Co₂B phases to form during solidification, indicating that not only the peritectic reaction of L (liquid) and Co₂B into Co₃B, predicted by the Co-B phase diagram, but also the formation of Co₃B as primary phase at large undercooling were inhibited. The peritectic reaction did not occur even though the solidification was designed to occur at a very small undercooling and a cooling rate decreased to 5 K/min.