The present study aimed to evaluate the effect of two mixtures of medicinal plants as a feed additives. The active compounds and effect on some blood serum parameters were studied. The first additive consisted of 1:1:1 Rosemary, Thyme and Peppermint mixture (RTPM). The second additive consisted of 1:1:1 Lemongrass, Parsley and Coriander mixture (LPCM). Twelve lambs of Saidi sheep of 5-6 months age, with an average weight of 20.49±0.76 kg were randomly allocated into three groups. The first group received control diet (concentrate feed mixture to roughage ratio; 70:30) without additives, the second group received control diet supplemented with 1.5% RTPM on dry matter (DM) basis, and the third group received control diet supplemented with 1.5% LPCM on DM basis. The experiment lasted for 159 days. All extracts of medicinal plants were analyzed using gas chromatography-mass spectrometry (GC-MS) for their bioactive compounds. The results showed that the principal compounds for RTPM were Eicosane, Heptane, 2,2,4,6,6-pentamethyl and Heneicosane for Rosemary; Tridecane, 1-iodo, Hexane, 2,3,4-trimethyl and Eicosane 10-methyl for Thyme and Hexadecane, 2-Bromo dodecane and Octacosane for Peppermint. The principal bioactive compounds for LPCM were Heptane, 2,2,4,6,6-pentamethyl-, Phthalic acid, ethyl neopentyl ester and Eicosane, 2-methyl for Lemongrass; Eicosane, Octadecane, 1-iodo and Docosane for Parsley and Octacosane, Docosane and Heneicosane for Coriander. The RTPM and LPCM ration increased serum total protein (P < 0.0002) and globulin (P < 0.0004); however serum albumin, urea, uric acid, creatinine, AST, ALT, cholesterol and glucose were not significantly affected. All values of blood serum constituents were within the normal ranges. The RTPM and LPCM feeding regimens led to decrease the rectal temperature (P < 0.0486) under the heat stress danger compared to the control diet.