The promoter effects of Zn and Sn in the direct synthesis of methylchlorosilanes were studied using a stirred bed. Zn and Sn were introduced not only before but also during the reaction. The activity and selectivity were characterized by online GC analysis and correlated with the Cu3Si content that was measured by the leaching method. It was found that Zn enhanced the formation of Cu3Si when it was added before the reaction. Zn improved the selectivity for dimethyldichlorosilane, but the excess Zn deactivated the reaction. Sn accelerated the consumption of Cu3Si. Added during the reaction, Sn led to a drastic reduction of Cu3Si coupled with a temporary rise in the formation of dimethyldichlorosilane. These results provided a new explanation for the role of promoters and suggested that the regulation on the direct synthesis process is probably due to the trade-off between the production and consumption of Cu3Si.