Based on a diesel-methanol dual-fuel engine, the effects of diesel injection timing and methanol substitution ratio on the combustion, emissions and fuel economy of dual-fuel engines at different loads were investigated. The results showed that the maximum methanol substitution ratio over diesel varied along with load condition. It was disclosed that relatively high methanol substitution ratio, with substitution ratio 75% to be the maximum, could be applied at low and medium loads. However, the results also disclosed that diesel injection timing had significant effects on engine performance. When diesel injection timing advanced, the maximum combustion pressure and thermal efficiency increased with shortened combustion duration. With injection timing -34°CA ATDC, indicated thermal efficiency reaches up to 41.7% when methanol substitution ratio is 55%. While, too advanced injection timing led to premixed combustion, thus sharp heat release rate and high peak pressure were obtained. To prevent mechanical failure, limited methanol substitution ratio was resulted in when too advanced diesel injection timing was adopted. With the increase of methanol substitution ratio, the maximum in-cylinder combustion pressure and the peak heat release rate increased with shortened the ignition delay and combustion duration. The nitrogen oxide emission decreased while the hydrocarbon and carbon monoxide emissions increased. At load BMEP 0.2MPa, the increase of methanol substitution ratio led to decreased indicated thermal efficiency. However, when load was as high as BMEP 1.0MPa, indicated thermal efficiency increased from 43% to 44% when methanol substitution ratio increased from 0% to 40%.