Structure effect on photochromic mechanism of 1,3,3 -trimethylspiro[indoline-2,3'-[3H]naphtho[2,1,-b][1,4]oxazine] (SP1), 1,3-Dimethyl-3,2'-(1,2' -propylene)spiro[indoline-2,3'-[3H] naphtho[2,1-b][1,4]oxazine] (SP2) and 1,2'-(1,3-propylene),3,3-dimethylspiro[indo- line-2,3' [3H]naphtho[2,1-b][1,4]oxazine](SP3) have been studied by nanosecond laser photolysis. Because of different connective positions of 1,3-propylene group in SP2 and SP3, the photochromic behaviours of them are quite different. One photoproduct of SP2 was only detected, and its lifetime is about 0.15 mu s. However, two colored forms of SP3 were detected after UV irradiation. The lifetimes of them are much longer than that of C-SP2. The semiempirical computational results by AMI method were coincided with the experimental results.