In the present work, four new 1,3,4-oxadiazole derivatives were synthesized. Two 2-substituted-5-[4-(benzamido)-phenyl]-1,3,4- oxadiazoles [4 and 5] were synthesized by a route in which 4-(benzamido) benzoylhydrazine [3] was reacted with benzoic acid to [ produce 2-phenyl-5- [4-(benzamido)-phenyl]-1,3,4-oxadiazole [4], and with propionic acid to produce 2-ethyl-5-[4-(benzamido)-phenyl]- 1,3,4-oxadiazole [5]. The 2-(aryl)-5-[4- (aryloxy)-phenyl]-l, 3,4-oxadiazoles [7 and 8] were synthesized by the reaction of 4-hydroxy benzoyl hydrazine [6] with (2) equivalents of 2-chlorobenzoic acid to produce 2-(2-chlorophenyl)-5-[4-(2-chlorobenzoyloxy)-phenyl]-1,3,4oxadiazole [7], and with (2) equivalents of 2-nitrobenzoic acid to produce 2-(2-nitrophenyl)-5- [4-(2-nitrobenzoyloxy)-phenyl]-l,3,4-oxadiazcle [8]. The synthesized compounds were identified.by their sharp melting points and FT-IR spctra. The effects of the synthesized oxadiazoles on the activity of - ribonuclease A (RNase A) were studied in vitro. Three of these : compounds showed inhibitory effects on the RNase A activity. Different concentrations of these compounds were used to study the type of inhibition. The results, which obtained from the Lineweaver-Burk plots indicate that [5] and [7] are competitive inhibitors, and [4] is a mixed type inhibitor.