A highly sensitive liquid-level sensor based on weak uniform fiber Bragg grating (FBG) with narrow-bandwidth is proposed and demonstrated. The resonance wavelength of FBG varies as a result of the change of axial strain. According to this characteristic, a precise approach utilizing fiber grating as sensing element to measure height of liquid is achieved. The grating is directly bonded to a float which can transfer buoyancy into axial strain on the fiber. Measurement of liquid level can be realized by monitoring the central wavelength shift of the sensing FBG. The experimental results show that for a liquid-level variation of 25 mm, a high sensitivity of with a good linearity of 0.991 is achieved. Within bidirectional change of liquid level, crossing sensitivity and long term stability taken into account, the system is of good reliability and stability.