Grabner made his international debut with Austria in Division I play of the 2004 IIHF World U18 Championships in Minsk, Belarus. Underaged for the tournament at sixteen-years-old, he scored three goals and an assist in five games. The following year, he competed for Austria in Division I of the 2005 World Junior Championships, held in Great Britain. He recorded three points in four games as Austria finished third in Group A. Several months later, he was named to his second under-18 team for the 2005 IIHF World U18 Championships, where Austria again competed in Division I play. Grabner tied for second in team-scoring with four goals and an assists over five games. Austria finished in fifth place out of six teams in Group A, coming within a loss of being relegated to Division II for the next year. He joined Austria in their qualifying tournament for the 2010 Winter Olympics in early 2009 and led the four team tournament in scoring with five goals; Austria finished second to Germany and failed to qualify for the Olympics.