With the center of research interest on selenium diverting from essential trace ele-ment to drug, birth of pharmacology of organo-selenium compounds will certainly beaccelerated. Selenosemi-carbazide has been demonstrated to have tumor-growthprohibiting and lipid-peroxidate lowering effects. We have for the first time success-fully synthesized its derivatives,α-aroyl β-acylamino selenoureas, and demonstrated byEPR study that they have very powerful O_2 scavenger activity. With the center of research interest on selenium diverting from essential trace ele-ment to drug, birth of pharmacology of organo-selenium compounds will certainly be administered. Have welen to have tumor to growth inhibitor and lipid-peroxidate lowering effects. for the first time success-fully synthesized derivatives, α-aroyl β-acylamino selenoureas, and demonstrated by EPR study that they have very powerful O_2 scavenger activity.