This paper presents simulation study of reducing pollutant emissions of diesel buses. The simulation studies were performed on a D1146TI turbocharged diesel engine that equipped with an Exhaust Gas Recirculation (EGR) system combined with a Diesel Oxidation Catalyst (DOC) and a Diesel Particulate Filter (DPF) by using AVL-Boost software. The simulation process was researched at operating regulations of engine and the ECE R49 test cycle. The simulation results showed that the nitrogen oxides (NOX) and Soot emissions are significantly dependent on the recirculation gas rate, and diesel engine emissions on buses can be up toward Euro3 standard when equipped with an EGR system combined with DOC and DPF. These simulation results have provided important information that can usefully assist in the first step of the design process of emission reductidn system for diesel engine on these buses.