Photonic crystal fiber (PCF) has been studied intensively in the past decade owing to its potential in fiber optic communication and sensing applications. Many research interests have been attracted to the long period gratings (LPGs) and LPG based devices in PCFs in recent years. Because of the microstructured air holes, the effective modal index shows strong wavelength dependence, which will result some anomalous properties in such gratings distinguished with conventional fiber. However, the mode coupling characteristics have not been investigated in details and the systematic comparison with the single mode fiber (SMF) based LPGs have not been analyzed yet. Moreover, because of the existence of the air holes in the cladding, we can include different aqueous solutions in such holey structure, such as pure water or sugar solutions. Meanwhile, the refractive index of these inclusions can be easily tuned by changing the temperatures, which will result in an evolution of the cladding modes. So some interesting coupling effects can be expected between the core and cladding modes. In this paper, the evolution of mode coupling with the structure change is discussed firstly. Then the attention is focused to investigate the PCF based mechanical LPGs with aqueous solution inclusions. The shift of resonance wavelength with the solution brix and the temperature is evaluated both theoretically and experimentally. This grating device offers the unique advantages of being tunable, removable, reconfigurable and strain-stable. These properties guarantee it to be utilized as a stable candidate for temperature and refractive index sensing.