Romeo's tragic flaw is his impetuosity, his rashness of action before thinking thoroughly. You would want to look for examples of this behavior. Examples include:
- Falling in "love" with Juliet within minutes of meeting her, even after finding out that she was a Capulet.
- Forgetting about Rosaline within minutes of meeting Juliet, though he had been depressed for weeks due to his unrequited love.
- Leaving his friends after the Capulet ball and climbing back into the Capulet compound, where he would be killed if discovered.
- Swearing his love for Juliet during the balcony scene, and asking her to marry him.
- Lying to the friar to get him to marry the couple by implying that he had sex with Juliet the night before. The friar then felt obligated to "erase" that sin by marrying them after the fact (as well as to end the feud).
- Holding Mercutio back before Benvolio had the chance to do the same with Tybalt, allowing Tybalt to stab Mercutio "under [Romeo's] arm."
- Killing Tybalt in anger despite knowing the consequences, which should have been execution.
- Threatening to kill himself with a dagger after the friar informs him that he is banished, not sentenced to death.
- Buying poison from the apothecary, bullying him into accepting the money though the sale of poison was illegal and punishable by death.
- Killing Paris without even knowing who he was. Paris had come to the Capulet tomb to innocently place flowers for his 'dead' fiance. He thought that Romeo was there to defile the crypt.
- Killing himself though he should have realized that Juliet was alive. After two days, Juliet shouold not have still had rosy lips and supple flesh. Romeo notices this, but cannot figure out that she must be alive. He rashly poisons himself though she awakened within minutes.
1. Romeo is impulsive! He acts without thinking. He professes love to Juliet after only knowing her for a few hours and he kills Tybalt impulsively, and has to live with thinking about it for the rest of the play.
2. As it goes with being impulsive, he falls in and out of love quickly. At the beginning of the play, Romeo professes that he is in love with Rosaline. But, at the mere sight of Juliet, Romeo falls head over heels for her.
There are many instances in which Romeo is impulsive, and this is his major tragic flaw as our tragic hero in Romeo and Juliet.
- The balcony scene: Romeo is rash and impulsive as he professes his love to Juliet.
-Forgetting Rosaline. Romeo easily, and impulsively, forgets a woman he is nearly betrothed to.
-Killing Tybalt. Romeo allows his emotions to take over and kills Tybalt in the healt of the moment.
-Killing himself. Romeo didn't even check to see if Juliet was alive, and he didn't think about consequences of killing himself. Instead, he impulsively kills himself, causing the death of Juliet.
I hope this helps :)