To provide an electrical connection box mounting structure which can surely perform the positioning of the electrical connection box.

At the time of fixing a relay box body 10 to a panel 25 through the screwing, a positioning means between the mounting holes 19, 23 is formed with an engaging structure of the projected area and recessed area, and the projected area (positioning projection) 24 is provided to a nut 21 mounted to the side of panel 25. Since the positioning projection 24 is mounted to the side of panel 25 as described above, the positioning projection 24 is never deformed in the course of process to carry in the relay box body 10 into the assembling field. Accordingly, reliable positioning can be performed at the time of mounting the relay box body 10.