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A novel formation method of long-period fiber gratings (LPFGs) based on periodic microchannels, which are fabricated by femtosecond laser micromachining and selective chemical etching in conventional single-mode fibers, is proposed and experimentally demonstrated. This kind of grating may be as short as only 3 mm and exhibits low-temperature sensitivity of 9.95 pm/°C from 30 °C to 120°C. The strain sensitivities of the resonant wavelength and peak loss are -2.4 nm/mε and -3.5 dB/mε, respectively. High refractive-index (RI) sensitivity with one order of magnitude larger than that of the conventional LPFGs near RI=1.33 is achieved in general single-mode fibers.