Abstract: The mechanism and potential energy surface of the addition reactions of H₂S and  CH₃SH to silastannene were studied by using density functional theory (DFT). The obtained results show that all reactions consist of a multi-step mechanism and the initial step of the reactions is a nucleophilic attack of H₂S or  CH₃SH toward silastannene. The formation of Si-S or Sn-S bond is always prior to that of Sn-H or Si-H bond in all studied reactions. The reactions involving a forming Si-S bond are more favorable than those containing a forming Sn-S bond kinetically, which is determined by energy factor.  CH₃SH as a nucleophile is more reactive toward silastannene than H₂S