In this letter, the chemical vapor deposition grown single-layer graphene with micrometer scale graphene flakes interspersed on the surface has been first used to implement field-effect transistors (FETs) for the purpose of frequency tripling. Two Dirac points can be observed in the transfer curves of the designed FETs, which are essential to generate pure third harmonic with ambipolar graphene. With an input of 1 kHz, more than 94% of the output signal RF power is concentrated at 3 kHz. To the best of our knowledge, it is the highest output spectral purity for reported frequency triplers. The excellent spectral purity, combined with the superhigh carrier mobility of graphene, makes this graphene-based frequency tripler a very promising candidate for the ultrahigh-frequency electronic applications.