It is shown that Witten's star product in string field theory, defined as the overlap of half strings, is equivalent to the Moyal star product involving the relativistic phase space of even string modes. The string field ψ A (x µ [σ]) can be rewritten as a phase space field of the even modes A (x µ 2n , x 0 , p µ 2n) , where x µ 2n are the positions of the even string modes, and p µ 2n are rela...