In this paper, effects of diameter on characteristics of a LPFG with numerical simulations adopting a three-layer model and experimental demonstrations are presented, including the normalized coupling coefficient, the effective refractive index, the resonant wavelength, the shape of the attenuation dip and the refractive index sensitivity. The couplings between the core mode and the EH cladding modes increase faster than that of the HE cladding modes as the fiber diameter decreases. Moreover, the depth and bandwidth of the attenuation dip vary faster in thinner fiber due to the increase of the coupling coefficient, comparing with LPFGs in common fiber. The resonant wavelengths of lower order cladding modes move mainly toward the shorter wavelength, while that of higher order cladding modes move mainly toward the longer wavelength as a function of the exponential shape as the fiber radius reducing and the external refractive index increasing. The refractive index sensitivity is greatly enhanced when reducing the fiber diameter and using the taper structure. Therefore, we can utilize the higher sensitivity of different cladding mode at a suitable wavelength by controlling the diameter of the fiber taper waist. Furthermore, the sensing resolution and the cladding mode can also be selected by controlling the fiber diameter, which can be greatly used in many fields.