This work aimed to examine the possibility of creating natural gas urban bus fleets by applying lean burn technology. Different engine configurations were tested, keeping in consideration the necessity to ensure suitable performance, and to meet the European regulations. With the target torque and power, the severe European limits were not met only for methane emissions. In addition, the GWI (global warming impact) values were also computed and compared with proposed limits expressly conceived for natural gas engines. The results showed that the NG lean burn engine at the present state of development does not appear able to meet the future requirements of both low NOx emissions and GWI, because of the difficulty of resolving the trade-off between NOx and HC.