High order resonances between fundamental core mode and cladding LP(01) supermodes are demonstrated in long period fiber gratings (LPFGs) inscribed in all-solid photonic bandgap fibers for the first time to our knowledge. The resonance wavelengths of the LPFGs calculated by way of photonic bandgap theory agree with the experimental results. The temperature responses of these resonance peaks have been theoretically and experimentally investigated. In addition, the mechanism of LPFG formation has been researched deeply through coupled-mode theory (CMT) and the cutback experiments.