The fight over Michael Jackson's death is NOT over, because Katherine Jackson has just appealed one of the judge's major decisions in the case ... TMZ has learned.

As you know, the jury ruled AEG was NOT responsible for MJ's death, concluding the company was not wrong in hiring Dr. Conrad Murray. Earlier in the case, the judge dismissed two key players as defendants ... CEO Randy Phillips and Paul Gongaware, who was in charge of producing and promoting Jackson's "This Is It" concert tour.

Katherine and MJ's 3 kids sued for more than $40 billion, claiming the loss of Michael devastated their lives -- and now apparently believe the decision to dismiss Phillips and Gongaware torpedoed their case.

Katherine and co. are appealing the dismissal.

It's very difficult to overturn judge decisions in civil cases, but it can happen ... especially if the judge screwed up an important ruling.