The purpose of this study is to improve the carbon monoxide (CO) and total hydrocarbons (THC) conversion efficiency of a diesel oxidation catalyst (DOC) by enhancing the monolith thermal behaviour through modification of the substrate cell density and wall thickness. The optimisation is based on catalyst properties (light off performance, conversion efficiency, pressure drop and mechanical durability).These properties were first estimated using theoretical equations derived from literature in order to select commercially available substrates for further modelling studies.The thermal behaviour and conversion efficiency of the selected catalysts under diesel exhaust gas conditions were numerically studied using data from an EU5 diesel engine operating a New European Driving Cycle (NEDC). This simulation was carried out on a commercial exhaust aftertreatment modelling program, AXISUITE. The predictions were compared to a reference coated 400/4 catalyst.The modelling shows that high cell density monoliths with reduced wall thickness (e.g. 600/2.5) have better catalyst light off performance and conversion efficiencies over the simulated NEDC. However during decelerations, these performances are limited due to thin walls undergoing rapid cooling. Conversely, catalysts with higher thermal mass (e.g. 200/12) reach lower peak temperatures but keep residual heat for longer.To limit these thermal effects, combinations of two catalysts in series were simulated, using a primary catalyst with good light off performance and a secondary catalyst with higher thermal mass to retain heat. The results show an overall improvement in the conversion efficiencies compared to the 400/4 single part while creating a limited back pressure increase.