Reformed exhaust gas recirculation (REGR), which can generate onboard hydrogen-rich gas (i.e., the reformate including H-2, CO, unreformed hydrocarbon, etc.) via catalytic reforming of fuel and engine exhaust gas, is an attractive way to improve the performance and emissions characteristics of the engine fueled with liquefied natural gas (i.e., NG engine). However, the leakage during the valve overlap period and incomplete burning of the added reformate may lead to extra HC and CO emissions from the engine with REGR. In the present study, a multi-dimensional computation fluid dynamics model coupled with a detailed chemical kinetic mechanism was developed to investigate the effects of the ratio of reformate addition (R-ref) and exhaust valve closed (EVC) timing on the total emissions characteristics as well as the sources of HC and CO emissions from the engine. The emissions from the combustion and the leaking were included to calculate the total emissions. Moreover, the unburned CO from the added reformate was distinguished from the total CO emissions by adding marked-species. Results show that the unburned CH4 in the cylinder is the main component of the total CH4 emissions. Due to the increase of the concentrations of OH, O and H radicals during the combustion process, the oxidization of CH4 is promoted with the increase of Rref at high load, and therefore the total CH4 emissions decrease. However, the total CO emissions increase with the increase of Rref, and it is demonstrated that the unburned CO from the added reformate increases and turns to be the main sources of the total CO emissions. At Rref of 10%, the total CH4 and CO emissions firstly remain nearly constant and then increase dramatically with the delay of EVC timing. Therefore, low concentration of CO in the reformate and short valve duration are recommended to achieve low HC and CO emissions for the NG engine with REGR. (C) 2018 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.