Background & Objectives: In a conventional method for determination of the chemical oxygen demand (COD), organic compounds are oxidized by potassium dichromate in acidic condition and thermal digestion (150°C, 2 hr). Whereas microwave produces heat through rapid rotation of polar molecules and ionic polarization, microwave digestion was investigated for rapid measurement of COD in present study. Methods: In this experimental study, in order to compare the thermal and microwave digesters for COD determination, 243 standard samples of potassium hydrogen phthalate solution and raw wastewater and effluent samples were studied according to standard methods based on the colorimetry of reduced chromium ion. The results were analyzed by Wilcoxon test. Results: The mean COD obtained from microwave digestion (360 Watt, 5 min) had no significant difference with the one determined by thermal reactor (150 °C, 2 hr.). Energy consumption of thermal reactor and microwave for determination of COD in each sample was determined as 736 and 30 W.hr, respectively. Therefore, the energy consumption and digestion time of thermal reactor in COD determination were about 24 fold higher than the microwave digestion. Conclusion: Application of microwave is preferred to the thermal reactor in terms of the time and energy consumption due to its catalytic ability in chemical oxidation.