In 2004, Kostitsyn had also made his senior international debut as a 19-year-old at the 2004 IIHF World Championship. He scored six points in five games. The following year in 2005 IIHF World Championship, however, he did not record a point in six games playing in the top division. Following his rookie season in the American Hockey League (AHL) in 2006, he played in his third consecutive World Championship and recorded five points in six games. At the 2008 IIHF World Championship, Kostitsyn recorded three points in five games.