A high sensitivity refractive index (RI) sensor based on a multimode interference arises from the self-imaging effect in single-mode–multimode–single-mode fiber structure was experimentally demonstrated. The sensor performance with different coreless fiber diameter (CLF) was examined to obtain an appropriate dimension of extreme evanescent fields. Various diameters of CLF of 100 and 50 μm with three different tune lengths 20, 25 and 30 mm were achieved by chemical etching technique based on hydrofluoric (HF) acid plunging. The sensor performance is recognized by determining the spectral shift from the transmission spectrum for different values of refractive indices. The highest sensor sensitivity of about 1012 nm/RIU was attained when the CLF diameter was 50 μm. This sensor offers an appropriate and economical technique for applications of RI measurement.