Abstract: The periodic boundary conditions structure of polydimethylsiloxane (PDMS) with two repeat units is full optimized using B3LYP/6-31G(d) method and the photodissociation of PDMS is studied using ab initio molecular dynamics (AIMD) method. The result shows that the hydrogen atom attached to methyl is the easiest cracking type in the bond dissociation process. Through the whole trajectory of PDMS, the C—H bond is the first one to disassociate, which produce H radical and silicone-methyl radical. And then the C—Si bonds break up into methyl radical and main chain radical. The (calculated) result agrees with the gamma radiolysis gases of PDMS in our early experiment data.