We report the development of photo-crosslinked siRNA strands modified at each terminus with p-cyanostilbene. The siRNA was nuclease resistant and retained RNAi activity. We further studied the activation mechanism of the covalently-crosslinked siRNA. Interestingly Dicer, which is known to generate siRNA with overhanging 3' ends from the precursor siRNA, did not cleave the crosslinked siRNA at all. Our results suggest that the activation of the crosslinked siRNAs required cleavage by Argonaute2.