Characteristics of cladding modes in a photonic crystal fiber (PCF) with triangular air-hole lattice in the cladding are numerically analyzed using a finite element method. The transition for cladding mode to core mode with variation of the normalized wavelength has been shown. The transition of the cladding mode to the outer silica mode and reorganization of the cladding modes caused by varying the fiber radius has been investigated. By choosing the optimized fiber radius, which is located in the cladding modes’ reorganization region, the sensitivity of the coupled wavelength between the core mode and cladding mode to surrounding refractive index is increased by a factor of five and reaches to index unit over the range of 1.40 to 1.42. The sensitivity is competitive with that of long-period grating in PCF in response to changes in refractive indices of the medium contained in the cladding air channels.