PYRIMIDINE and fused heterocyclic pyrimidine derivatives are of great biological interest, especially as antiviral, antitumor and antimicrobial agents. The viruses are one of the most threatening factors for plants resulting in gigantic economic losses. Eight chemical compounds belonging to the fused heterocyclic pyrimidines were screened in order to assay their antiviral activity as well as determining their mechanism of action against two plant viruses TMV and TNV by using the local lesion assay on Datura metel for TMV and Phaseolus vulgaris for TNV. Only two compounds out of eight compounds (1 and 8) showed significant percentage of inhibition for both viruses at concentration 20 μg/ml while the remaining six compounds (2, 3, 4, 5, 7 and 9) showed weak inhibitory activity at same concentration. Mechanism of action studies on both potent compounds revealed that compound number (1) had virucidal activity against both TMV and TNV while compound number (8) seemed to affect the replication process of both viruses.