A series of 4-substituted-5,6,7,8-tetrahydro [1] benzothieno [2,3-d] pyrimidine derivatives (6a-f) were prepared by the displacement reaction between various amines and 4-chloro-5,6,7,8-tetrahydro [1] benzothieno [2,3-d] pyrimidine (5), which was obtained by refluxing 5,6,7,8-tetrahydro [1] benzothieno [2,3-d] pyrimidin-4 (3H)-one (4) with phosphorus oxychloride. Compound (4) was obtained by cyclization of ethyl 2-amino-4,5,6,7-tetrahydro-1-benzothiophene-3-carboxylate (3) with formamide. Compound (3) was obtained by refluxing cyclohexanone (1), sulphur and ethyl cyanoacetate (2) in ethanol. The title compounds were screened for antibacterial activity. Compounds 6a and 6b were found to have excellent antibacterial activity against Staphylococcus aureus and Streptococcus pneumoniae when compared with standard used (amoxicillin-clavulanic acid).