Heavy equipment trailer and deck extension. The trailer has an elongated main deck with longitudinally extending frame members on opposite sides thereof, and ground engaging wheels toward the rear of the main deck. The extension has longitudinally extending frame members aligned with the frame members of the main deck, and coupling pins extend between the frame members of the main deck and the extension in load supporting relationship, with lock pins preventing longitudinal separation of the main deck and the extension. C-shaped channel members extend along the upper, lower, and outer sides of the extension, past the coupling pins, and along upper, lower, and outer side portions of the frame members of the main deck to hold the frame members in alignment and prevent sagging under load conditions.