The pilot diesel injection timing (θ) has a dramatic influence on the combustion and performance of dual-fuel (DF) engines. In order to optimize the θ of a natural gas-diesel DF engines, the influence of θ on combustion, cyclic variation and performance of a diesel engine fueled with natural gas piloted by diesel under full load at 1200r/min was investigated. The results indicate that, with advance in θ, the cylinder pressure, rate of pressure rise and heat release rate increase first and then decrease. The mean value of peak cylinder pressure (pmax) rises and the standard deviation increases first and then decreases. The distribution of crank angle of peak cylinder pressure (φ(pmax)) scatters and approaches the top dead center. The coefficient of variation (COV) in pmax decreases first and then increases while the COV in φ(pmax) obviously increases. The brake power increases first and then decreases while the b.s.f.c. reduces first and then rises. The CO2 and NOx emissions rise first and then reduce while smoke emission decreases first and then increases, but the CO and HC rise.