The initial decomposition of methylamine on Mo(100) surface has been investigated by self-consistent (GGA-PW91) density functional theory combined with periodic slab model. The adsorption energies of possible species and the activation energies for possible elementary reactions involved are obtained in the present work. Our results indicate that the barriers decreased with the order of C-N>N-H>C-H. In addition, metastable adsorption of the abstracted hydrogen atom on the hollow site in the final state is also considered for the N-H and C-H bond breaking. For the C-H bond cleavage, the reaction barrier that the abstracted hydrogen located on the hollow site in the final state is lower than that on the bridge site. However, for the N H bond breaking, the barriers are alike for the abstracted hydrogen on both hollow and bridge sites in the final state. The initial decomposition of methylamine on Mo (100) surface has been investigated by self-consistent (GGA-PW91) density functional theory combined with periodic slab model. The adsorption energies of possible species and the activation energies for possible elementary reactions involved are obtained in the present work. Our results indicate that the barrier decreased with the order of CN> NH> CH. In addition, metastable adsorption of the abstracted hydrogen atom on the hollow site in the final state is also considered for the NH and CH bond breaking. For the CH bond cleavage, the reaction barrier that the abstracted hydrogen located on the hollow site in the final state is lower than that on the bridge site. However, for the NH bond breaking, the barriers are alike for the abstracted hydrogen on both hollow and bridge sites in the final state.