A mounting mechanism for fixing a computer mainframe includes a retaining base and an article; the retaining base includes a first plate, a second plate and an extensible screw; the retaining base is coupled to the article; the first plate is disposed with at least one hanging tab; the lower end of the first plate is coupled to the second plate, and the hanging tab is positioned above the second plate in order to couple to a computer mainframe with a different width conveniently; the hanging tab may be extended in a hanging hole of the computer mainframe corresponding thereto; a bolt of the extensible screw may be extended out of a hole of the second plate to engage with a screw hole of the computer mainframe corresponding thereto thereby allowing the computer mainframe to be coupled to or detached from the mounting mechanism easily without using tools.