A series of new 5-imino-4-thioxo-2-imidazolidinone derivatives 3 with various halogenated and alkylated aromatic substituents at N1 and N3 was synthesized. Imidazolidineiminothione derivatives 3 were prepared from the reaction of N-arylcyanothioformamide derivatives with aryl isocyanates. These compounds were used as key synthons for the preparation of wide variety of new substituted imidazole compounds. Imine hydrolysis of 3 with ethanolic HCl produced the corresponding 4-thioxo-2,5-imidazolidindiones 4. Condensation of 3 with benzophenonhydrazone furnished the corresponding 4-azine derivatives 5. Monohydrazono and dihydrazono derivatives 6 and 8 were obtained upon treatment of imidazolidinone derivatives 3 with hydrazine hydrate. Finally, imidazolidinones 3 were reacted with o-phenylenediamines or pyrazol- 5(4H)-ones and afforded the corresponding imidazoquinoxaline and imidazolidin-4-ylidenepyrazolone-5(4H)-one derivatives 11 and 12, respectively. Evaluation of the antibacterial and antifungal activities for the synthesized compounds was carried out to probe their activities. Most of the tested compounds showed significant activities. The best antimicrobial activity was observed for 1-(3-ethoxyphenyl)-6- methyl-1-phenyl-1H-imidazo[4,5-b]quinoxalin-2(3H)-ones (11c) followed by 5-imino-3-(3-methoxy- phenyl)-1-phenyl-4- thioxoimidazolidin-2-one (3f).