Using the tightly focused visible light (wavelength λ=488nm) illuminating, the ferroelectric domain patterns of the undoped lithium niobate crystal have been demonstrated. The influence of the visible light intensity on the domain nucleation field was investigated. The reduction of nucleation field decreases exponentially with increasing incident irradiation intensity. Once a domain is nucleated it can be dictated by the far-field light diffraction patterns. An assumption is proposed that the reduction of nucleation field is directly related to the defects mobility and structure of the crystals.