The cold start performance of a diesel engine has been receiving more attention as the European Commission emission regulations directed to include cold start emissions in the legislative emission driving cycles. The cold start performance of diesel engines is influenced by the ambient temperature conditions, engine design, fuel, lubricant and engine operating conditions. The present research work investigates the effect of cold ambient conditions on the diesel engine's performance and the exhaust emission (gaseous and particulate emissions) characteristics during the cold start and followed by idle. The engine startability and idling tests were carried out on the latest generation of diesel engine in a cold cell at various ambient temperatures ranging between +20°C and −20°C. Higher fuel consumption and peak speed were observed at very cold ambient compared to those at normal ambient during the cold start. The engine exhaust emissions were reached a peak value during the acceleration period of the cold start, and it decreased to maintain the idle speed. The exhaust emissions during the cold start and idle conditions were higher at very cold ambient conditions compared to normal ambient temperatures. The exhaust particle size distribution analysis indicated that the particle number concentration has been shifted towards larger diameter particles with a drop in ambient temperatures. The particulate mass at idle conditions was about 1/10th of that cold start for the all the temperature conditions. The relationship between the particle number and particulate mass during cold start and idle is also established. The cold start performance of the engine at very cold ambient conditions significantly affects the engine transient performance as well.