An attempt was made to optimize NOx and particulate emissions from heavy-duty diesel powered vehicles under heavy load engine operating regions by combining EGR and methanol fumigation and the effects on exhaust emissions were experimentally studied.The results under steady states tests show that, the smoke concentration is decreased and total fuel consumption is improved according to the increase in methanol energy ratio. As NOx reduction effect of EGR does not affected by methanol fumigation, drastic NOx reduction can be thereby possible at heavy load regions with the combined use of EGR and methanol fumigation.Then, this method was applied to new Japanese 13 mode test procedure and it was recognized that the NOx mass emissions were reduced to almost one half without increase in particulate emissions.However, drastic increase in CO, HC and aldehyde emissions were observed also.EXHAUST GAS RECIRCULATION (EGR) is one of the probable counter measure for NOx emission from heavy-duty diesel powered vehicles. We have already presented that the effective NOx mass reduction is possible without significant increase in other pollutants or fuel consumption by means of limited EGR operations under light and medium load engine operating regions(1).However, at heavy load regions, use of EGR is difficult because of the drastic increase in smoke or particulate emissions. In case of heavy-duty vehicles, the frequency of operations at heavy load regions is increasing according to the growth of pay-load or average speed. Therefore, avoiding the increase in smoke emission, NOx reduction effect by the EGR might be restricted to insufficient level, and consequently, the alternative means should be considered for NOx reduction at heavy load regions without increase in other pollutants.On the contrary, it is well known that introduction of methanol in a diesel engine can have an advantage concerning smoke emissions over a use of conventional diesel fuel. No smoke emissions can be achieved even at heavy load regions. Then, if control of smoke emissions at this region could be possible by means of auxiliary methanol introduction, the use of EGR at this region may become possible and effective NOx reduction could be realized.The objective of this study is to investigate possibilities of NOx reduction from heavy-duty diesel engines without significant increase in other pollutants under engine heavy load operating regions by the combination of EGR and auxiliary methanol introduction.