Co-worker stealing work and credit

I work part time as an office assistant.

I am having a a problem with another full time employee who's current positon does not have enough work for her to do everyday. My boss is unaware that her workload has decreased.

After I left for the day she took one of my completed assignments changed it slightly and presented it to my boss and took credit for it. This is not the first incident.

How would you suggest I handle this situation? I would not want her to take over my position and find myself out of work.

I'm on your side here, but I feel obligated to prepare you for the worst: If your co-worker is full time and doesn't have enough to do, the most logical thing to do is eliminate the part time position. On that cheery thought, here is what I would do.

I would quietly approach your co-worker and say, "Jill, when you took credit for my work, that was not cool. Not cool at all. Please don't do it again."

Say it calmly and attempt to walk away after you say it. Why? This isn't a discussion you want to have. Jill will be defensive and tell you how she made a ton of changes and your work was terrible and she was saving you from embarrassment by taking over the project. It doesn't matter that none of this is true.

Then I would let that particular incident go. If it happens again, I would go to your boss with a copy of your draft of the project and say, "I know I'm not here all the time, but Jill keeps presenting my work as her own. Here's a copy of how I left the information. I don't want to cause problems with Jill, but I also don't want you to think I'm not working hard."

Then let your boss take it from there.

Unless Jill is a brazen type of person, I doubt she'll do it again after you've told her in plain language that it is unacceptable. People do things like this because they can get away with it. Being called out on it is usually enough to get all by the most egregious offenders to stop.