Photonic crystal devices have been regarded as the key solution for overcoming the limitations of conventional optical devices. In recent years, their novel and unique features have been investigated and exploited by many researches. This work presents a powerful design tool based on Finite Element Method, FEM, to be applied in the design and optimization of these devices, such as Photonic Crystal Fibers (PCFs) and integrated optical waveguides with photonic crystals. The full vectorial FEM code which has been implemented can be used for accurate modal and dispersion analysis. Some examples are presented concerning a novel approach for designing large-mode-area endlessly single-mode PCF and optical waveguides with photonic crystals with reduced critical power of the optical bistability phenomenon. © Springer-Verlag 2004.