We report a theoretical and experimental investigation of the dispersion property of long-period fiber gratings in B/Ge codoped fiber. A novel concept of a mode ultrasensitive zone defined by a region in which the phase-matching conditions are close to the dispersion turning points is proposed. The transition from a dual-resonance state to a dispersion turning point is revealed for a set of ultrasensitive-zone gratings during the UV-writing process. We explore the sensitivity characteristics near the dispersion turning points, with the aim of implementing two types of temperature sensor with sensitivity 2 orders of magnitude higher than that of the conventional long-period gratings.