As a player for the Yugoslavia national basketball team Demšar participated in 1950 World Championship and three European Championships, 1947 in Prague, 1953 in Moscow and 1955 in Budapest. He played 79 games for national team. During EuroBasket 1947, on 13 May 1947 he score 42 points on 90–13 win against Albania national team and set national team scoring record.