Novel ultra-long period fiber gratings (ULPFGs) with periods of up to several millimeters, which were written by the high-frequency CO_2 laser pulses,were reported.The refractive index change occured mainly within the cladding area of the fiber and had an unsymmetrical distribution on the cross-section of the ULPFG.The resonant loss peaks of the ULPFG were generated from the coupling of the forward propagating core mode to the cladding modes of fundamental and harmonic orders.Such a ULPFG can be used to measure multi-parameters simultaneously as these resonant loss peaks have different sensitivities to temperature,strain,bending and torsion,et al.The temperature and strain characteristics of the ULPFGs with 3 mm period were investigated by experiment.Meanwhile,several new approaches of the potential applications of the ULPFG were proposed.