The stability and electronic structure of Si6H4Ph2 are investigated by a comparative study of pure silicon nanosheet Si6, hydrogen-passivated silicon nanosheet Si6H6 and phenyl-passivated silicon nanosheet Si6H4Ph2 using density functional calculations. The mechanism on the stability of Si6H4Ph2 is elucidated. In addition, by examining the electronic structures of Si6H6 and Si6H4Ph2, we find the they both behave like an indirect gap semiconductor with a quite large gap.