Information is lacking on the specific biological activity of feeds containing different types of condensed tannin (CT) on rumen fermentation characteristics. The aim of this study was to evaluate the in vitro rumen fermentation parameters of diets including sainfoin (Onobrychis viciifolia Scop.) pellets (PS) and/or hazelnut (Corylus avellana L.) pericarps (HP) using a batch culture system for 24 h. The treatments were a basal diet consisting of (dry matter (DM) basis) 800 g/kg hay from permanent grassland and 200 g/kg concentrate mix (control), the basal diet + 30.4% PS, the basal diet + 8.2% freeze-dried HP, and the basal diet + 15.2% PS + 4.1% HP. The diets were adjusted to be isotannic (20 g/kg DM, except for the control) and isoproteic (132 g/kg DM). Total gas and methane (CH4) productions were measured after 3.5 h and 24 h of incubation in buffered rumen fluid from sheep. At the end of incubation, pH, in vitro DM degradability (IVDMD) and the concentration of fermentation end-products in the medium were also measured. The CT structures in PS and HP, determined by the thiolysis method, were very different PS had mostly prodelphinidins and HP mostly procyanidins. After 24 h of incubation, the total gas and CH4 productions and IVDMD were greater for the basal diet than for the CT-containing diets (P < 0.001). The CH4 production increased significantly with the diet + HP in the presence of polyethylene glycol (PEG, 4000 Da molecular weight), a CT-inactivating compound (P < 0.001), and tended to increase for the diet + PS (P = 0.062). The volatile fatty acid (VFA: acetate, propionate, butyrate, minor and iso-VFA) net productions were similar among treatments except for valerate (the lowest for PS-containing diets, P = 0.003), while the NH3 concentration was lower for the diet + PS (with a significant PEG effect) than for the diets including HP, and was highest for the basal diet. It was concluded that the inclusion of PS and HP in a basal diet for ruminants results in lower rumen fermentability and that their CT decreased CH4 production and protein degradability. The PS were more effective than HP for reducing rumen protein degradability with a potential increase of duodenal nitrogen (N) flow.