Thermally tunable photonic crystal fibers (PCF) are proposed by filling the air hole of PCF core with nematic liquid crystal (LC) and using the LC refractive index change with temperature and wavelength, due to index sensitive to temperature and wavelength. The effect of temperature on PCF transmission properties is studied by the step effective index model and numerical calculation. The results show that dispersion decreases in the PCF infiltrated with liquid crystals, and the PCF appears as single-mode transmission in long wavelength but not in short wavelength, and non-cutoff-wavelength single-mode transmission does not appear even if relative hole diameters in cladding are very small. The dispersion increases and zero dispersion wavelengths shift towards short wavelengths when temperature increases in the PCF. The properties are valuable for design and application of temperature tunable PCF device.