An environmentally benign transparent photochromic film, 1,3-dihydro-1,3,3-trimethylspiro[2H-indole-2,3'-[3H]-naphtho[2,1-b][1,4] oxazine] (SPO) /gamma-cyclodextrin (gamma-CD), was prepared using the cast-coating method, and the nanocavity effect of gamma-CD on the photochromism of SPO was studied. The film mainly consists of the inclusion complex SPO@gamma-CD, which has been verified by EA, TGA, XRD, MS, and ICD to comprise a 1:1 host-guest stoichiometry. The film shows normal photochromism. The decoloration of photomerocyanine (PMC) fits biexponential decay: PMCs located in the cavity of gamma-CD decay with a rate constant of 6.0 x 10(-2) s(-1), which is nearly one order faster than those PMCs outside of the cavity.