Try LSAT logical reasoning questions.

duongthang, did you ever find an answer to your question? would you mind sharing what you learned about hard CR questions?

duongthang wrote:we can find hard math questions, hard RC passages and hard SC

how we know and find hard CR questions. I think that if we find many hard CR and solve them, we can master CR section.

try kaplan! kaplan questions are always harder than questions in another sources

sometime I meet a hard CR question. But I do not know why it is hard, where to find it.

LSAT CR questions contain FORMAL LOGIC which is not tested in GMAT. I will try LSAT but will keep in mind that some fomal logic related questions should not be studied.

duongthang wrote:we can find hard math questions, hard RC passages and hard SC

Hi duongthang. Use the GMATFix Solutions Engine to search for questions. You can pick question type (CR, RC, SC, PS, DS), topic (CR Weaken, Geometry...) and difficulty level (up to 700+). The engine will return matches to your search parameter; all questions in the engine are from GMATPrep. You can begin your search here.

Hope that helps,
-Patrick[/quote]

If you have already exhausted the GMAT materials then a good place to look is old LSAT exams - ignore any questions dealing with principle or parallel reasoning and know that these questions are probably much harder than most you will see on the GMAT but it is a good source.

And is doing the "hard" questions useful?

I dont want to sound sarcastic, but want to know for myself. I am targeting a score of around 730. Should I practice LSAT CRs

yes, LSAT questions will certainly be useful - you just don't want to put too much emphasis on the more formal logic questions - that is why you would eliminate principle, parallel and any question that says "if this then that and if not that then another thing" type questions - those aren't on the GMAT. Good Luck