The aim in this study is to develop the combined EGR system with a non-thermal plasma reactor for reducing exhaust emissions and improving fuel economy in turbo intercooler ECU common-rail diesel engines. At the first step, in this paper, the characteristics of performance and $NO_x{cdot}THC$ emissions under four kinds of engine loads are experimentally investigated by using a four-cycle, four-cylinder, direct injection type, water-cooled turbo intercooler ECU common-rail diesel engine with a combined plasma exhaust gas recirculation(EGR) system operating at three kinds of engine speeds. The EGR system is used to reduce $NO_x$ emissions, and the non-thermal plasma reactor and turbo intercooler system are used to reduce THC emissions. The plasma system is a flat-to-flat type reactor operated by a plasma power supply. The fuel is sprayed by pilot and main injections at the variable injection timing between BTDC $15^{circ}$ and ATDC $1^{circ}$ according to experimental conditions. It is found that the specific fuel consumption rate with EGR is increased, but the fuel economy is better than that of mechanical injection type diesel engine as compared with the same output. Results show that $NO_x$ emissions are decreased, but THC emissions are increased, as the EGR rate is elevated. $NO_x$ and THC emissions are also slightly decreased as the applied electrical voltage of the non-thermal plasma reactor is elevated. Thus one can conclude that the influence of EGR in $NO_x$ and THC emissions is larger than that of the non-thermal plasma reactor, but THC emissions are greatly influenced by the non-thermal plasma reactor as the EGR rate is elevated.