Femtosecond laser writing is a powerful three dimensional (3D) engineering technique for materials processing. It has been used to modify the refractive index of dielectric materials (e.g., glass, crystals) to construct optical waveguides for diverse applications. Lithium niobate (LiNbO₃) is a multifunctional crystal with excellent nonlinear optical properties. Nonlinear photonic crystal (NPC) could control nonlinear optical interactions through quasi-phase matching (QPM) due to the periodical nonlinear coefficients. Although the 1D and 2D NPCs have been fabricated by electric-field poling, the implementation of 3D NPCs remains as a major challenge in nonlinear optics because of the limitation of traditional poling methods.