Recent developments in diesel engines lead to increased fuel efficiency and reduced exhaust gas temperature. Therefore more energy efficient aftertreatment systems are required to comply with tight emission regulations. In this study, a computational fluid dynamics package was used to investigate the thermal behaviour of a diesel aftertreatment system. A parametric study was carried out to identify the most influential pipework material and insulation characteristics in terms of thermal performance. In the case of the aftertreatment pipework and canning material effect, an array of different potential materials was selected and their effects on the emission conversion efficiency of a Diesel Oxidation Catalyst (DOC) were numerically investigated over a driving cycle. Results indicate that although the pipework material's volumetric heat capacity was decreased by a factor of four, the total emission reduction was only considerable during the cold start.Different insulation strategies (e.g. double layer pipe with air gap and vacuum) were simulated using CFD and the improvement in the DOC emission conversion was monitored over the New European Driving Cycle (NEDC). It was found that insulation strategies can considerably improve the aftertreatment performance particularly during engine high-load conditions. To improve the cold-start performance a new aftertreatment pipework design was developed and simulated to examine its performance in terms of thermal behaviour during different engine operating conditions. This design introduces a well-insulated pipework system with minimized thermal inertia to accelerate the aftertreatment catalyst light-off.