The variation relationships among resonant wavelengths, fiber parameters and grating period of long-period fiber gratings (LPGs) are studied. It is found that the high order cladding modes are quite different from the low order ones. Their resonant wavelengths are shifted toward opposite directions as the parameters vary. Ihis difference was confirmed by fabrication of two LPGs with period of 100 μm and 400 μm respectively. Some useful results for design and fabrication of LPGs are obtained.