This paper describes that it applies to diesel truck engines with the tandem Pre-Turbocharger Catalysts (PTCs) set before the turbocharger. At first, it confirmed the advantages of the PTCs compared to a conventional Diesel Oxidation Catalyst (DOC) size and position. Then, it also confirmed that new emission reduction process hypothesized by tandem very small PTCs. Moreover, metal structure design substrate (LS-Design) is applied to the PTCs system. As a result, we have insight of the possibilities of multiple / tandem PTCs systems instead of the conventional DOC for truck applications with new-generation diesel engine and aftertreatment system.