Abstract: Although the content of 1,3,5-trimethylbenzene is high in the isomerization of 1,2,4-trimethylbenzene, the process is extremely difficult to be realized industrially because of the accumulation of methylethylbenzene. A unique process using isomerization and alkylation as a union has been studied, in which alkylation is used to relieve the accumulation of metylethhylbenzene whose by-product can be used as parts of raw materials in isomerization to control the side reactions, and isomerization can raise the content of 1,3,5-trimethylbenzene. Thus 1,3,5-trimethylbenzene and the coproduct 1,2,4,5-tetramethylbezene can be produced industrially. Moreover, the production cost can be lowered greatly. Meanwhile, the effects of technological conditions on the reactions have been discussed, and the optimum technological conditions offered. The technology has now been used industrially to produce 600t/a 98.5% 1,3,5-trimethylbenzene.