This's third time that I play FF9. I tried to steal Mythril Sword from her. So I didn't hurt her. But about 10 round, she used Stock Break to end the battle even she wasn't lost HP anymore. Can you tell me what happen?

I think one of the battles or more is time based if you go over that time then you get shock break. But I think they are more than just 10 rounds more like 20 odd rounds.

Just go back to the save moogle, and perhaps level up the characters so the speed is faster and Zidane can steal leads more and he'll have a better sucess rate. Speaking of sucess rate turn on Master Theif (Theif's Gloves) and Bandit (Mythril Dagger and N-Kai Armlet) so you have a better chance of stealing and better chance of stealing rare items. If you mean the first time you encounter Betrix then it's a 1/256 chance of getting it without these abilities, with I don't know the chance but it's a lot.

Yeah, Beatrix's battles are also time-based. So the battle ends if the time runs out, or if you take out all of her HP, whichever comes first.

You probably don't have those abilities yet for Zidane, so it would be wise to equip the equipment that raises Zidane's Spirit stat as high as possible. A higher spirit stat helps with stealing.

I must have been lucky cause i stole it on my first try

Yeah, Beatrix's battles are also time-based. So the battle ends if the time runs out, or if you take out all of her HP, whichever comes first.

You probably don't have those abilities yet for Zidane, so it would be wise to equip the equipment that raises Zidane's Spirit stat as high as possible. A higher spirit stat helps with stealing.
You can get the Mythril Dagger in Treno, so you should be able to use Bandit

You can get the Mythril Dagger in Treno, so you should be able to use Bandit

OK, thanks for point that out. I don't exactly remember the locations of whichever weapons have those abilities. :)