All of the Law and Order series stories are fictional. However, their theme, stories 'ripped from the headlines' is often true in that they take a real story from the news and make their own, fictional version of it. I often recognize the real story that an episode came from. The stories are often very much the same in the beginning of the Law and Order episode, but always takes twists and turns that end the story unrelated to the real one.
Many of the episodes are based on real cases, especially on the original Law & Order. There are two books out that actually recount the real stories that the episodes are based on. The books are called True Stories of Law & Order. They are really interesting.