BACKGROUND: Recently, the researches about herbal medicines as substitutes for chemical drugs have risen in number. OBJECTIVES: To add more aspects to this research area, this study evaluated the changes in biochemical factors of the blood serum due to the consumption of Thymus daenensis extract and the effect of the period of its consumption into the diet of herd dogs. METHODS: Eight adult male dogs were chosen and fed with a fixed diet for 1 month. Then they were ran- domly assigned to two groups of four. The members of first (control) group were fed only with the fixed diet and the second group received 200mg/ml, administered as 1ml/kg of Thymus daenensis extract into their fixed diet. Finally, the dogs were bloodlet at 3 different times, namely before intervention, 7 and 14 days after the in- tervention to measure the factors of glucose, cholesterol, triglyceride, LDL and HDL, alanine aminotransferase and aspartate aminotransferases. RESULTS: Comparing to the control group and the starting point of this experiment, levels of FBS, LDL, TGL, CHOL, AST factors significantly decreased 14 days after intervention, but the HDL factor increased after 7 and 14 days of intervention (P 0.05). CONCLUSIONS: Consequently, oral consumption of Thymus daenensis extract led to decrease in the amounts of glucose and lipids of blood and this herb can be recommended for treating diabetics and persons suffering from high cholesterol.