The mechanism and kinetics of unimolecular decomposition of CH_{3}SO_{3} are studied at the G3XMP2//B3LYP/6-311+G(3*df*,2*p*) level of theory. Six possible dissociation channels and potential energy surface for the CH_{3}SO_{3} decomposition are investigated. Rate constants over the temperature range of 200-3000 K are calculated using Rice-Ramsperger-Kassel-Marcus (RRKM) theory. The results indicate that the product P1(CH_{3}+SO_{3}) is dominant between 200-3000 K. Products P2(CH_{3}O+SO_{2}) and P3(HCHO+HOSO) increase significantly at higher temperatures (>3000 K). Products P4(CHSO_{2}+H_{2}O), P5(CH_{2}SO_{3}+H) and P6(CHSO_{3}+H_{2}) show little formation in the temperature range (200-3000 K). The total rate constant can be expressed as *k*_{total}=1.40×10^{12}*T*^{0.15}exp(7831.58/*T*). Thermodynamic properties including enthalpies of formation (D_{f}*H*^{Θ}_{298 K}, D_{f}*H*_{0 K}), entropies (*S*^{Θ}_{298 K}), and heat capacities (*C** _{p}*, 298-2000 K) of all the minima and transition states are predicted from statistical mechanics, and found to be in good agreement with the available experimental values.