In situ observation of the ferro-elastic transition in BNN crystal is carried out with the convergent polarized interference pattern, it reveals that the crystal transforms from bioaxies to uniaxies in heating with an inflection point Ts at 250±20℃. The transition is of the continuous and diffuse type. A large hysteresis about 20-30℃ is show by the fact that heating and cooling curves are distinct above 210℃. The twins (ferro-elastic domain) disappear and reappear in accordance with the time of phase transition. The density of the twins influences the inflection point Ts of the phase transition and the degree of hysteresis leading to decrease in Ts and increase in the hysteresis. The defects and microscopic compositional inhomogeneities are the main cause for the diffuse of the phase transition.