Methyl-substituted benzenes and methyl-substituted naphthalenes are important starting chemicals for organic synthesis and polymer production and superior additives of high-density fuels. This work indicates for the first time that they can be synthesized from acetone and calcium carbide (CaC2) in one pot at 200-350 degrees C. As reported in the literature on acetone conversion, 3,5-xylenol is simultaneously formed with the methyl-substituted benzenes. At the optimized conditions, the total carbon yield of methyl-substituted benzenes and methyl-substituted naphthalenes reaches 48% and the carbon yield of 3,5-xylenol is about 20%. The role of CaC2 and the synthetic pathway of methyl substituted benzenes and methyl-substituted naphthalenes are investigated through identification of intermediates, control experiments, and quantum chemical calculation by density functional theory (DFT). It is found that methyl-substituted benzenes and methyl-substituted naphthalenes are formed via tandem reactions involving acetone condensation and intermediates aromatization. CaC2 promotes acetone condensation and its alkynyl moiety directly participates in aromatization reaction to form methyl-substituted naphthalenes, which are attributed to the stronger nucleophilicity of the alkynyl moiety. This protocol enables the direct synthesis of methyl-substituted benzenes and methyl-substituted naphthalenes from acetone and CaC2, and opens up a safe and efficient route for CaC2 application.