Not Sachin but Clark was the first cricketer to hit a double century in ODI matches

Sachin Tendulkar was not the first cricketer to score a double century in ODI matches but Australian women cricketer Belinda Clark was. Belinda Clark is former Australian cricketer who played international cricket for Australia's women national team from 1991 to 2005. She was the first ever cricketer to score a double century in One Day International matches when she scored 229* against Denmark women's team in 1997 world cup. She came to bat as an opener and scored an unbeaten 229 runs in 155 balls with the help of 22 fours and became player of the match as well.

In men's ODI matches, Sachin Tendulkar is the first batsman to score a double century. He scored 200* runs vs South Africa in 147 balls with the help of 25 fours and 3 sixes.