Microstructures of 0.18wt% nitrogen-contained duplex stainless steel, SUS329J3L brazed in a vacuum furnace of 10-4 torr, have been investigated as a function of bonding temperatures (1453-1523K) and holding times (0-1.8ks). An amorphous alloy, MBF50 (Ni-19.5wt%Cr-7.3wt%Si -1.5wt%B), was used as an insert metal. At an early stage at 1453K and 1473k, a morphology change of the insert metal, BN and CrB phase appeared only at the joints. The BN and Cr-Si-N phase were observed at the interface of the joints brazed under other conditions. The volume fraction of BN increased rapidly at an early stage and decreased with increasing holding time. The phase seemed to have been formed by dissolution of the base metal and the diffusion process. BN was formed easily due to the lowest Gibbs free energy. Boron content in liquid insert metal becomes low due to the formation of a large number of BNs at the bonded interlayer by holding for a few minutes at brazing temperature. This caused the rapid isothermal solidification of the liquid insert metal. Thus, it is clear that the isothermal solidification process of this bonding is controlled by the formation of boron nitrides as opposed to the diffusion process of depressant elements(B and Si) in the base metal.