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To achieve diesel engine ultra-low nitrogen oxide emission, light-off selective catalyst reduction (LO-SCR) has been suggested for better performance with lower exhaust temperature. An electric heater upstream of the exhaust aftertreatment system was applied to significantly decrease the NO_x emission at a low exhaust temperature. With a 7.2 kW electric heater coupled with LO-SCR, the NO_x emission during 200~500 s of the world harmonized transient cycle (WHTC) decreased from 282.6 ppm to 61.5 ppm, which is a decrease of 45%. Application of an upstream diesel oxidation catalyst (DOC) decreased the NO_x emission by 63% at the same interval at the cost of worse cold-start performance. The urea input was also adjusted to avoid NO_x emission during the latter part of the WHTC.