In this paper, we have proposed a semicircular curved hetero-core optical fiber accelerometer consisting of an optical fiber and a mass for low-frequency vibration monitoring. Experimental results showed that the proposed accelerometer with the mass installed had improved sensitivity in the low-frequency range and that differences in sensitivity occurred depending on the direction of the applied vibration. Sensitivities in the direction of in-plane shear deformation were 4.4×10^-1, 1.3×10^-1, and 8.1×10^-3 dB/g with 2×10^-4 kg, 7×10^-5 kg, and without mass, respectively. Experimental results showed that heavier mass improved sensitivity in the low-frequency range. Besides, the sensitivity in the out-of-plane bending deformation direction was lower than that in the in-plane shear deformation direction, with a transverse sensitivity of 15 to 25% compared with the in-plane shear deformation sensitivity. In addition, the frequency response characteristics obtained from the FE analysis and experiments revealed a trade-off between sensitivity and measurable frequency range depending on the mass condition.