Today the Ammonia Selective Catalytic Reduction (SCR) system with good NOx conversion is the emission technology of choice for diesel engines globally. High NOx conversion SCR systems combined with optimized engine calibration not only address the stringent NOx emission limits which have been introduced or are being considered for later this decade, but also reduce CO2 emissions required by government regulations and the increase in fuel economy required by end-users. Reducing the packaging envelope of today's SCR systems, while retaining or improving NOx conversion and pressure drop, is a key customer demand.High SCR loadings ensure high NOx conversion at very low temperatures. To meet this performance requirement, a High Porosity Substrate which minimizes the pressure drop impact, was introduced in SAE Paper 2012-01-1079 , , . The High Porosity Substrate with an equivalent catalyst amount demonstrated a pressure drop reduction in SCR substrate against the baseline conventional substrate. Moreover, high porosity substrate with high catalyst amount shows a possibility to achieve high NOx conversion and significant downsizing.As previously proven , , , High Cell Density is an effective way to increase catalyst surface area and NOx conversion at high temperatures due to the mass transfer effect. A High Porosity, High Cell Density Substrate has been developed which offers a notable improvement in NOx conversion over a wide temperature range as well as SCR substrate volume reduction.This paper will define the concept and SCR NOx conversion performance of the new High Porosity with High Cell Density Substrate compared to conventional substrate. The evaluation also includes pressure drop, durability, and potential SCR downsizing.