The dynamics of the two unimolecular reactions that initiate the thermal decomposition of methyl nitrite were investigated by direct dynamics calculations. The two decomposition pathways are (I) O-N bond scission to form CH3O and NO and (Il) concerted elimination through a four-center transition state to produce CH2O and HNO. Structural data along the reaction paths were obtained from high-level ab initio methods. Specifically, the elimination reaction path was achieved from MP2 results scaled so that the height of the barrier coincided with the value given by QCISD(T)//QCISD calculations. The dissociation path was first calculated at the CASSCF(8,8) level of theory and then scaled to reproduce the dissociation energy predicted by QCISD(T)//CASSCF(8,8) computations. All the ab initio calculations were performed with the standard 6-311++G(d,p) basis set. Thermal rate constants were evaluated by canonical variational transition-state theory(CVT). For the elimination process, tunneling was taken into account by using the approximations zero curvature tunneling (ZCT) and small curvature tunneling (SCT). The overall agreement between the calculated rate constants and the experimental ones reported in the literature is reasonably good. The calculations indicate that the dissociation is remarkably faster than the elimination not only because the barrier height for the O-N bond scission is lower than that for the elimination reaction but also because the former process is entropically favored.