In order to examine the nature of the silyl radical in the disilaanthracene framework, separation and radical isomerization of the cis and trans isomers of the disilaanthracene derivatives were investigated. When a pentane solution of the disilaanthracene derivative was irradiated with a 400 W mercury lamp in the presence of DTBP for 4 h, both isomers readily isomerized to give a mixture of cis and trans isomers in a ratio of ca. 50:50. However, no such isomerization reaction of 9,10-dihydro-9-silaanthracene derivatives occurred under similar conditions.