The mechanisms of multiphoton ionization(MPI)and dissociation of CH₃I at 355 nm are studied under supersonic beam conditions with time-of-flight mass spectrometer. MPI mass spectrum consists mostly of fragment ions,H⁺,CH(+,3)and I⁺. In addition,weak signals of C⁺,CH⁺,CH(+,2),and CH₃I⁺ are also observed in the spectrum. This is consistent with the parent-ion ladder mechanism in which the parent molecules are two-photon resonantly excited to Rydberg C state and then ionized through an additional one-photon absorption to form CH₃I⁺. Fragment ions are produced by dissociating CH₃I. According to the appearance potential of the parent and fragment ions,the main possible channels of the MPI and photo dissociation of CH₃I are analyzed.