We reported a long period grating (LPG) written in specially designed photonic crystal fiber (PCF) for refractive index (RI) sensing by interrogating the transmitted light power. The outermost ring of clad holes of the PCF is enlarged where the analyte is filled. We showed that the leakage loss of the clad mode increases with the RI in the larger holes. By numerically analyzing the complex couple mode equations for the core mode and the first clad mode, we found the depth of attenuation band in the transmitted spectra and the total transmitted power is sensitive to the leakage loss of the clad mode or the RI in the larger holes. We also demonstrated that the transmitted power is sensitive to the RI even less than that of the silica, which just avoids the limitation that the transmitted light power of LPG in conventional fiber is only sensitive to the RI of the external media higher than that of fiber clad.