The molecular geometry of nine halogenated monoterp enoids from the red alga plocamium cartilagineum in the ground state has been calculated by B3LYP/6-31G*. F uroplocamioid C (1), pre furoplocamioid (2), pirene (3) and compound 5 had selective effect against cancer cell s versus CHO cells. The gap energy of compounds 1, 2, 3, and 5 ranges from 5.6 to 6.1 and is lower than that of th e compounds 4, 6, 7 and 8.Thus the gap energy plays an important role towards selective activity. The stereochemical features of the compounds also play an important r ole to activity