Time series analysis of diesel exhaust gas emissions under transient operation was carried out using a uniquely developed gas sampling system to efficiently collect all exhaust gas throughout transient cycles. The effects of fuel properties and other engine operation parameters on the exhaust emissions under transient runs when fuel amounts abruptly increase were analyzed.The results showed that THC increased abruptly to 2 or 6 times the final steady-state concentration immediately after the start of acceleration and then decreased to the steady-state values after 70∼200 cycles. At acceleration, NOx increased abruptly to about 80 % of the final NOx concentration, and then increased gradually to reach the final values after 60∼500 cycles. The behaviors of THC and NOx during transient operation can be described by exponential functions of the elapsed cycle numbers and the final emission concentrations. The concentrations of CO, CO2, O2, and smoke changed abruptly to the steady-state values at the start of acceleration. The dominant factors affecting the gas emissions during transient operations are the deposition of fuel on the combustion chamber walls and the combustion rapidity.