To evaluate the bioaccessibility of phenolic acids in extruded and nonextruded cereal grains, an in vivo experiment was carried out using growing pigs as a model system. Four diets were prepared containing either whole grain barley (BU), dehulled oat (OU), or their respective extruded samples (BE, OE) according to the requirements for crude protein, mineral, and vitamin contents in pig diets. The total contents of free phenolic acids in the OE and BE diets were 22 and 10%, respectively, higher compared with the OU and BU diets, whereas the level of bound phenolic acids was 9% higher in OE than in OU and 11% lower in BE compared with BU. The total tract bioaccessibilities of bound phenolic acids were 29 and 14% higher for the extruded BE and OE diets, respectively, compared with the nonextruded diets. The results of this study indicate an improved bioaccessibility of phenolic acids in extruded cereal grains.