An experimental study is carried out to compare the effects of high-pressure-loop, low-pressure-loop and dual-loop exhaust gas recirculation systems (HPL-EGR, LPL-EGR and DL-EGR) on the combustion characteristics, thermal efficiency and emissions of a diesel engine. The tests are conducted on a six-cylinder turbocharged heavy-duty diesel engine under various operating conditions. The low-pressure-loop portion (LPL-Portion) of DL-EGR is swept from 0% to 100% at several constant EGR rates, and the DL-EGR is optimized based on fuel efficiency.The results show that the LPL-EGR can attain the highest gross indicated thermal efficiency (ITEg) in the three EGR systems under all the tested conditions. At a middle load of 0.95 BMEP, 1660 r/min, the pumping losses of LPL-EGR lead to the lowest BTE among the EGR systems. The HPL-EGR can achieve the best brake thermal efficiency (BTE) and emissions within the EGR rate of 22.5% mainly due to the reduced pumping losses. For the DL-EGR, the low-pressure loop is enabled when the EGR rate exceeds 22.5%, and the LPL-Portion should be increased as the EGR further increases. The DL-EGR can achieve the best BTE and emissions. This is because the DL-EGR can achieve an optimum trade-off performance between the ITEg and pumping losses, and the lowest emissions are related to the proper ignition delay and equivalence ratio. As the engine load decreases, the low-pressure loop in DL-EGR mode should be activated at a higher EGR rate. At a lower speed of 1330 r/min, 1.43 MPa BMEP, the HPL-EGR can achieve a slightly better BTE within the EGR rate of 12%. The DL-EGR can achieve the best BTE for the higher EGR rates, but the emissions of LPL-EGR are much better than that of DL-EGR.