Abstract: The mechanism of methyl indan formation during tetralin pyrolysis was investigated by using the density functional theory (DFT). The results showed that 1-methyl indan is the main conformation of methyl indan products from tetralin pyrolysis. As the main route of 1-methyl indan formation during tetralin pyrolysis, β tetralin radical was first formed through H abstraction by radical species from tetralin, which then endures a ring contraction reaction to form 1-methyl indan. High temperature can promote the formation of 1-methyl indan, but has little effect on the formation routes of 1-methyl indan.