It was found that addition of boron improved room temperature fracture strength by formation of Ni3Al(gamma') thin layer around the NiAl(beta) phase after heat-treatment in the NiAl+Ni3Al two phase alloy. Directional solidification technique with quenching of the solidifying interface has been applied to understand the mechanism of formation of the gamma' layer around the beta phase. The quenched solid/liquid interface showed that solidification ended up with gamma phase at the beta interdendritic region in the boron added alloy, while the gamma' phase formed at the interdendritic region in the binary alloy. It is thought that the gamma' phase formed around the beta dendritic phase grows between gamma and beta phases by a diffusion couple manner in the boron added alloy. It was also interesting to find that boron addition changed the phase field and solidification behavior.