Using first principles calculations within the generalized gradient approximation, the structural stability of compound Mo-S clusters at subnanometer level is discussed based on results obtained for various Mo 5Sn isomers with n = 5-15. The symmetrical Mo-S bonds generally lead to higher stability, while the formation of S-S bond and the observation of magnetism in these clusters lower the structural stability. Mo5Sn isomers with n = 6, 8 and 11 are found to be most stable. The addition of sulfur atoms is quite favorable in the edge capped square pyramid and face capped trigonal bipyrammid isomers. {\textcopyright}2007 Society of Nano Science and Technology.