Optical guiding characteristics of photonic crystal fibers (PCF) can be tuned by varying the parameters of air holes (diameter, distance and arrangement) and filling dielectric materials. Liquid crystal is one promising material used as the filling agent in PCF due to its electrical tunability, which can lead to the electrically tunable PCFs. Finite element method (FEM) is used to explore the effective refractive indices and effective areas of fundamental modes of PCF filled with liquid crystal (E7) as functions of duty cycles and voltages. Both the cut off voltages under variable duty cycles and cut off wavelengths under constant voltages are obtained for fundamental modes. Numerical results also show that cut off voltages increase as the duty cycles increasing, meanwhile the tunable wavelength range decreases as voltages increasing under constant duty cycles. PCFs filled with liquid crystal are expected in the application of electric field sensing.