I just got a new dog, she fights with my cat

Well if she's just trying to play, don't punish her for it. Does the cat hiss and swat at the dog? I own a dog and three cats and my dog is the same way; he always tries to play with the cats. I'm thinking the cat will punish the dog on its own. If I were you, I'd just tell the dog no, but nothing too severe. The dog is just trying to make friends and just doesn't understand how big she is.

Don't punish, just assert the law, LEAVE THE CAT ALONE. Put her leash on so that you can control her and make her lay down not facing the cat. With many dogs, I would let the cat train the dog, but border collies can be killers if challenged and it wouldn't take much for her to attack the cat and badly hurt it or kill it. A border collie is bred to protect the herd and they can kill a small animal so fast there is nothing you can do to stop it. You don't want this to become something it isn't. Right now it's a game but a dangerous one.

These tips might be really helpful for you and your situation: http://www.paw-rescue.org/PAW/PETTIPS/DogTip_Cat.html My boyfriend's mom has a cat, and they got a new puppy earlier this year. It was pretty crazy at first... but now the two of them get along just fine. So don't worry, in time it will get better.

My 2mos old pup plays with my older dog and they bite each other. I knew they're just playing specially the pup, but still bothers me and scared that my older dog might kill the pup. XD