The emission legislations are becoming increasingly strict all over the world and India too has taken a big leap in this direction by signaling the migration from Bharat Stage 4 (BS 4) to BS 6 in the year 2020. This decision by the Indian government has provided the Indian automotive industry a new challenge to find the most optimal solution for this migration, with the existing BS 4 engines available in their portfolio. Indian market for the LCV segment is highly competitive and cost sensitive where the overall vehicle operation cost (vehicle cost + fluid consumption cost) is the most critical factor. The engine and after-treatment technology for BS 6 emission levels should consider the factors of minimizing the additional hardware cost as well as improving the fuel efficiency. Often both of which are inversely proportional.The presented study involves the optimization of after treatment component size, layout and various systems for NOx and PM reduction. The study aims to analyze hardware changes required on the selected BS 4 engine to achieve improved engine out emission levels so as to minimize the Exhaust gas After-Treatment System (EATS) size using 1-D simulations approach.Normally the after treatment calibration and the engine thermodynamic calibration are performed separately which leads to complexity achieving the optimum fuel consumption and tail pipe emissions at vehicle level. FEV has developed a new strategy in which simultaneous optimization of involved systems like engine level thermodynamics, controls, transmission system and the EATS can be performed, thus arriving at the best engine and EATS configuration.In this study, the approaches to migrate from BS 4 to BS 6 for a Light Commercial Vehicle (LCV) application have been detailed. For an existing BS 4 diesel engine there can be two different approaches possible to meet the BS 6 emission targets. The first approach involves keeping the existing engine hardware unchanged and arriving at the required EATS, layout and sizing. The second approach involves implementation of engine level internal measures for improving engine-out emission levels and then arriving at a more optimized EATS. The engine level changes considered for this approach does not include any major architectural changes in the hardware. Both the approaches are evaluated on these two factors: a) cost of hardware addition to the existing engine and after-treatment system b) operational fluid cost. No changes into the transmission layout have been considered in this study. Although it was established that increase in the number of gears in the transmission will help reducing the engine out emissions .