Indigestible neutral detergent fibers (iNDF) accurately predict forage digestibility when measured in situ. The objective of this study was to determine the effects of rumen incubation times on the estimated concentrations of iNDF for four forages (alfalfa hay, corn silage, wheat straw and orchard grass), four concentrates (barley grain, soybean meal, beet pulp and wheat bran) and two total mixed ration samples in dairy cows. The iNDF contents of the samples were evaluated in 10 feeds using three ruminally cannulated Holstein cows in a completely randomized design. Five grams of the samples were incubated up to 240 hr. The iNDF fraction was significantly affected by incubation time for all of the tested samples, but the potentially digestible NDF fraction (pdNDF) was not affected for wheat straw, barley grain and wheat bran (32.32, 10.11 and 20.60 g per 100 g of dry matter, respectively). For most of concentrates feedstuffs, the iNDF fraction could be measured after 120 hr of incubation, while for forages ruminal incubation should be lasted up to 240 hr. Statistically significant differences (p < 0.01) were observed between forage samples regarding fragility and NDF digestibility (NDFD). Also, a positive correlation was observed between fragility and NDFD. In some of the cases, it appears that NDFD can be a more helpful index in adjusting pdNDF values than direct fragility measurements.