The purpose of the present study was to determine the chemical composition and in vitro ruminal fermentation capacity of potato pulp (PP), potato pulp with peel (PPp) and potato peel (Pp), i.e. potato starch by-products. PP and PPp were applied at 5.0, 7.5 and 10.0% (on a dry matter basis) instead of corn flake and barley grain in dairy cow total mixed ration (TMR). The volumes of in vitro total gas and methane production, and metabolisable energy (ME) values of PP and PPp were higher compared to Pp (P < 0.001). The in vitro true dry matter digestion (tDMd) and in vitro true organic matter digestion (tOMd) values of PP using at 7.5% and 10% rates in TMR were decreased linearly according to that of control TMR value (P < 0.05). Therefore, the use of potato starch by-products in ruminant diet can be suggested due to the high and rapid fermentation potential, especially of potato pulp without peel. The study showed that PP could be applied at up to 5% and PPp at a level of up to 10% in dairy cattle TMR (on a DM basis). Further in vitro studies should be carried out using dried potato starch by-products.