Abstract: A 1,2,3,5-tetrahydroimidazothienopyrimidin-2-one represented by the formula: ##STR1## wherein one of Z.sub.1, Z.sub.2 and Z.sub.3 is a sulfur atom and the remaining two of Z.sub.1, Z.sub.2 and Z.sub.3 represent CH, R.sub.1 and R.sub.2 each represents a hydrogen atom, an alkyl group having 1 to 5 carbon atoms, a phenyl group, a chlorine atom or, when taken together R.sub.1 and R.sub.2 represent an alkylene chain of 3 to 5 carbon atoms, and R.sub.3 represents a hydrogen atom or an alkyl group having 1 to 5 carbon atoms, and a pharmaceutically acceptable acid addition salt thereof, which exhibit an excellent blood platelet anti-aggregatory activity and are useful as anti-thrombotic agents.