The objective of this study was to evaluate the effect of palmitic, stearic and oleic acid on NDF digestibility and rumen fermentation. Continuous culture fermenters (n = 8) were used in a replicated 4 × 4 Latin square design with 7 d of adaptation and 4 d of sampling. Treatments were: 1) control diet without fatty acids; 2) control diet plus 1.5% of palmitic acid (99% C16:0); 3) control diet plus 1.5% of stearic acid (99% C16:0); and 4) control diet plus 1.5% oleic acid (99% cis-9 C18:1). The control diet (60 g DM/day) was a 50:50 orchardgrass hay:concentrate mixture that provided 8.7 g CP, 21 g NDF, 11 g starch, and 1.5 g fatty acids fed twice daily. The fatty acid treatments maintained the same nutrient input into the fermenters as the control except for fatty acids. Daily fermenter effluent was collected over 24 h post-feeding and a 30% subsample was pooled by fermenter within period. Buffer solution was delivered continuously at rate of 10%/hour. Data were analyzed using a mixed model including the fixed effect of treatment, and the random effects of period and fermenter. Data are reported as least squares means with differences declared at P ≤ 0.05 and tendencies at 0.05 < P ≤ 0.10. Compared with control, oleic acid tended to decrease NDF digestibility (36.6% vs. 43%, P = 0.09), whereas no effect was observed for palmitic (46.6% vs. 43%, P = 0.33) and stearic acid (44.5% vs. 43%, P = 0.68). Palmitic (P = 0.01) and stearic (P = 0.04) increased NDF digestibility compared with oleic acid; whereas no difference (P = 0.57) was observed between palmitic and stearic acid on NDF digestibility. Compared with control, total production of VFA was not affected (P > 0.10) by the fatty acid treatments (161.4, 168.9, 151.0, and 146.7 mmol for control, palmitic, stearic, and oleic, respectively); however, palmitic acid increased VFA production (P = 0.03) compared with oleic and tended to increase (P = 0.08) compared with stearic acid. Ammonia concentration was not affected by fatty acids (P > 0.10), nor was molar proportion of VFA (P ≥ 0.40). In conclusion, palmitic and stearic did not affect NDF digestibility nor VFA production compared with control diet, while oleic acid impaired both variables.