This report presents report calculation of extension frame design for Cometto trailer chassis connection. This analysis has been carried out with considering of the heaviest cargo 385 ton with additional consequence factor weight up to 150%. The material of frame is using steel ASTM 36 but material integrity check of the base frame using Non Destruction Check (NDT) are not part in this analysis. The report covers engineering scope pertaining to the transportation of the heavy cargo (module) with 385 Tons weight with dimension 24m length x 6.30m width where cometto multi-axle trailer configured with side by side and the extension frame will be assembled in between. The frame shall be adequate and strong enough to carry cargo and become one integrity with cometto trailer platform. This engineering calculation was requested by PT. Karga Walon Jakarta.