An expeditious process for selective nitration of 1-methyl-4-(methylsulfonyl)benzene in a continuous-flow reactor in 98% yield is described. The continuous-flow mononitration ran well in adiabatic condition with 80 wt% sulfuric acid, and residence time was reduced to 5 s. The short residence time can significantly reduce byproducts. Dinitration was also studied for optimization of the mononitration. The spent acid could be reused for several times after concentration in a vacuum.