The aim of this study was to investigate the effects of various inulin levels on in vitro true dry matter digestibility (IVTDDM) and in vitro neutral detergent fibre digestibility (IVTDNDF) of corn silage (CS), perennial ryegrass (PR), and common vetch/oat hay (VO). Inulin was added to the fermenter at concentrations of 0 (CSC, PRC, VOC), 100 (CS100, PR100, VO100), 200 (CS200, PR200, VO200), and 300 (CS300, PR300, VO300) mg/litre of total culture fluid using an in vitro DaisyII incubator. Each fermenter contained 1600 ml buffer solution and 400 ml rumen fluid. The IVTDDM and IVTDNDF were determined with a DaisyII incubator and rumen fluid obtained from three cannulated Karayaka rams. The IVTDDM values (%) for PRC, PR100, PR200, and PR300 were 70.06 ± 1.133, 73.21 ± 4.153, 70.36 ± 0.506, and 66.69 ± 1.317, respectively. The effects of various inulin levels on IVTDDM and IVTDNDF values of PR were significant (P <0.05). The IVTDDM and IVTDNDF values for CS and VO were not significantly (P >0.05) different. Among the treatments, supplementation of inulin to CS and VO did not have a significant (P >0.05) effect on IVTDDM and IVTDNDF values. The high dose of inulin (PR300) reduced IVTDDM and IVTDNDF, whereas PR100 showed a statistically significant (P <0.05) increase on IVTDDM and IVTDNDF. However, in vivo studies with PR may be required to show the effects of various levels of inulin supplementation to support the IVTD findings of the current study.