The fabrication mechanism of Long-Period Fiber Gratings(LPFGs) in standard SMF-28 telecommunication fibers without hydrogen loading were explored by using direct writing method based on femtosecond laser pulses, with pulse duration of 100 fs, 1kHz repetition rate and a central wavelength of 800 nm. The experimental system to write LPFGs is adopted directly the horizontal and vertical double CCD video to realize monitoring and alignment of fiber. The LPFGs with different spectral characteristics were fabricated by adjusting the number of grating period, energy density of laser irradiation and duty ratio. The result shows that resonance peak loss generate single-peak when the period of grating is constant, and the variations of the number of grating periods and pulse energy of laser irradiation will lead to the change of resonance peak loss. The variations of duty ratio will lead to the generation of resonance rejection band of LPFGs from single-peak to multi-peak plus lesser out-of-band loss. And finally, a resonance transmission spectra with 15 dB attenuation, 3 dB bandwidth of 15 nm, and less than 2 dB out-of-band losses at wavelength of 1 540 nm was obtained.