Reactions in solid state polymers are different from those in solutions because the mobility of the polymer molecules is so restricted that reactivities vary with the local environment of site by site. One of the influencing factors on their reactivity is the free volume in polymers. In this paper, free volume effect on the photo-induced molecular orientation reaction of azobenzene derivatives in poly(methyl methacrylate) (PMMA) was described and the distribution of the kinetic parameters is discussed in terms of the free volume distribution in PMMA.