In this paper, three S-tapered fiber (STF) structures with different diameters (40, 60, and 80 µm) are fabricated using conventional single-mode fiber. First, the reproducibility of the proposed S-tapered structure is confirmed through an analysis of the diameter distribution. Considering the transmitted intensities of the three various diameter, S-tapered structures reveal that the STF with a 40 µm diameter produces more evanescent waves and is more sensitive to external refractive index variations. Therefore, the STF structure with a 40 µm diameter was evaluated for the detection of different concentration of glucose solutions, demonstrating that the structure has the potential to be utilized to develop a highly sensitive fiber sensor.