In this paper, a common NOx model for hydrogen and diesel fueled combustion engines is developed and experimentally validated on a diesel engine with diesel and hydrogen/diesel dual-fuel operation system. The model is developed based on the simple measurement with the commonly used sensors situated on the engine system. NOx model is then calibrated with NOx sensor in diesel and hydrogen/diesel dual-fuel operation experiment with different engine variables, such as engine speed, engine brake power (equivalence ratio), hydrogen energy share ratio, injection timing and exhaust gas re-circulation (EGR). From results, it is observed that the model performed well in diesel engine with diesel fuel operation, injection timing and EGR with a single calibration factor on wider range of operating conditions. Moreover, NOx model has also shown satisfactory results with hydrogen/diesel dual-fuel combustion using a single calibration factor on a fixed operating condition (speed and brake power constant) with hydrogen ratio changes. The changes in calibration factor evident different fuel combustion properties and flame temperature kernel travel in cylinder changes for diesel and hydrogen fuel. Hence, finally, it can be stated that the developed NOx model can be used for diesel as well as hydrogen/diesel dual-fuel engines with simple calibration efforts. (C) 2018 Elsevier Ltd. All rights reserved.