0738名無しさん@英語勉強中 (ワッチョイ 99bd-c21i)2018/02/09(金) 01:51:09.15ID:5hSfR10X0 I'd say "I think it's not true" would be used when you are sure that it's false. And "I don't think it's true" would be more if you just kind of think that it might not be true.
In other words I'd use "I think it's not true" if I'm more confident that it's not true.