X-ray crystallographic analysis of the single crystals of endoperoxide of anthra[1,9-bc:4,10-b’c’]dichromene (1) has revealed the positions of singlet oxygen addition to 1. The sites of oxygen attack were 9- and 10-positions of anthracene moieties but not 1- and 4-positions, coincided with the prediction from the coefficients of highest occupied molecular orbital calculated by AM1 or PM3 method.