Cylindrical structures analysis is important in several areas and this can be performed through the analysis of the diameter changes of these structures. Two important areas can be mentioned: pipelines for oil or gas distribution and, condition and growth of trees. In the diameter tree changes, the monitoring is directly related to irrigation, since it depends on the soil water deficit and trees are important in the global circulation of heat and water. This diameter can change in the order of 5 mm for some species. In this paper, it is proposed and experimentally investigated a strain gauge sensor based on a core diameter mismatch technique for diameter measurement. The sensor structure is formed by splicing an uncoated short section of MMF between two standard SMFs called SMF-MMF-SMF (SMS), the MMF length is 15 mm. Two cylindric structures were developed on a 3D printer, with different diameter sizes (DS: 80 mm and 110 mm), to assist in monitoring the diameter changes. The SMS sensor was placed on the printed structure and fixed at two points such that by reducing the diameter of the structure, the sensor presents dip or peak shift of transmittance spectrum due to induced curvature and strain force. Two values were used for the spacing between the fixations points (FP): 5 mm and 10 mm. For DS = 80 mm, the sensor presents respectively: a) a sensitivity of -0.876 nm/mm, R² of 0.9909 and a dynamic range of 5 mm; b) a sensitivity of -0.3892 nm/mm, R² of 0.9954 and a dynamic range of 4 mm. For DS = 110 mm, the sensor presents respectively: a) a sensitivity of -0.22 nm/mm, R² of 0.9979 and a dynamic range of 8 mm; b) a sensitivity of -0.2284 nm/mm, R² of 0.9888 and a dynamic range of 6 mm.