A newly series of 6-(phenylurenyl/thiourenyl) saccharin (6a-y) derivatives were synthesized and their inhibitory effects on the diphenolase activity of banana tyrosinase were evaluated. A 70-fold purification of the enzyme with 6.85% yield was achieved by using a Sepharose 4B-l-tyrosine-p-amino benzoic acid affinity column. The result showed that all the synthesized compounds inhibited the tyrosinase enzyme activity. Among the compounds synthesized, 6-(3-iodophenylthiourenyl) saccharin (6s) was found to be most active one (K(i)=3.95 μM) and the inhibition kinetics analyzed by Lineweaver-Burk double reciprocal plots revealed that compound 6s was a competitive inhibitor. Structure-activity relationships study showed that generally, most of the 6-(phenylthiourenyl) saccharin derivatives (6m-y) exhibited higher inhibitory activity than 6-(phenylurenyl) saccharin derivatives (6a-l). An electron-withdrawing group at 3-position of phenylurenyl-ring increased in activity and the halogen series at 3-position of phenylthiourenyl-ring showed a qualitative relationship for higher inhibitory activity with increasing size and polarizability. We also calculated HOMO-LUMO energy levels and dipole moments of some selected the synthesized compounds (6a, 6h, 6m and 6s) using Gaussian software.