Yes, it says so on the warriors timeline it states 'In attempting to run away from the clans, Hollyleaf runs into an underground tunnel and is killed when the tunnel collapses'.
Click to see for yourself: 
Well she may have lived i Hope she lived because that would make a good story for the erins.
Hollyleaf dying is a question that isn't answered. While she was escaping the tunnel, the book clearly said that there was no evidence of her whatsoever, while the tunnel collapsed. Stated to the first person to answer, the cats assumed that she was dead, just like they assumed Graystripe when he went missing. But, again, it all comes back to the book. I hope she stays alive....She was my favorite character, and I didn't know what to say when I heard her death. She knew what she was doing, and she knew that she was doing it right! It was just that no one would understand her...
Look! Vicky and her friends are saying to refuses to tell fans what happened to Hollyleaf leading to rumors and theories about it. So, please forget about Hollyleaf and wait knowing about her. Please!
Yeah! I mean If she WAS dead they would of told us. I THINK she returns in Night Whispers.
Okay, if you say so.