Nanometric narrowband Long Period Gratings (LPG) are investigated for the implementation of improved fiber optic biosensors. The reduction of more than one order of magnitude in the linewidth of the LPG with respect standard LPG at 1500 nm, leads to the improvement of the resolution of the sensor. By selecting the proper fabrication parameters (high numerical aperture, relatively high order mode and large length), LPGs with a 3-dB bandwidth of 1.5 nm were fabricated. The sensitivity of the LPG as a refractometer was calculated, and experimentally characterized in detail. In particular, the LP0,17 and LP0,18 resonances were investigated, in order to select the most suitable one for the bioexperiments. The surface of the LPG was functionalized and the detection of the hybridization of DNA is demonstrated. When the biosensor was immersed in a 2 μM solution of the complementary DNA strand, the resonances of the LPG shifted in wavelength. When measuring the response of the sensor in terms of variation of its transmittance as the resonance shifted in wavelength, the sensitivity was ~10%/μM for both resonances. The detection limit was estimated in 10 nM