As a junior in 2006, Mattison he recorded 59 tackles, seven sacks, 11 tackles for loss, six pass breakups, four forced fumbles and one fumble recovery. The game against Ohio State was one of his best, as he recorded eight tackles and one forced fumble. He also performed well against Michigan, recording five tackles, two sacks and a forced fumble as the Hawkeyes lost to the number two team in the nation. Named honorable mention all-Big Ten by both the media and the coaches, his four forced fumbles were fourth in the Big Ten and his seven sacks ranked eighth in the Big Ten.