Tighter emission norms and fuel economy demands have prompted diesel engine manufacturers to implement Aftertreatment systems for both light-duty and heavy-duty diesel applications. After implementing Diesel Particulate Filter (DPF) technology to comply with 2007 Environmental Protection Agency (EPA) emissions regulations, OEMs have turned their attention towards NOx reductions with SCR technology. Current SCR technologies include liquid based Urea injection into the exhaust stream for NOx reductions and Solid Ammonia Storage and Delivery System (ASDS) which involves dosing gaseous Ammonia. Irrespective of the technology in use, the estimation of engine-out NOx emissions plays a vital role in reductant (Urea/Ammonia) dosing estimation via feed-back controls. The general method for determination of the engine-out NOx emissions is to use commonly available NOx emission sensors (NOx Sensors). However, NOx sensors have their own drawbacks. With On-Board Diagnostics (OBD) requirements in place for the 2013 model year, the use of additional sensors translates to additional implementation costs adding up to warranty and maintenance costs. Therefore, the necessity of the virtual engine out NOx emission estimation is very important. In this regard, a study has been performed to predict the engine out NOx emission values both at steady state and transient conditions. The NOx emission model uses the empirical relation that is based upon the real time estimation of NOx emissions. The empirical relation is a function of engine parameters such as engine speed, engine load, intake Oxygen concentration, injection quantity, timing, fuel pressure etc.