The accurate measurement of online reaction temperature in microwave devices has been reviewed by (Kappe, 2013). Infrared optical pyrometers can be used to measure the pyrolysis process temperature. However, these devices can only reflect the temperature of outer surface of pyrolysis sample. Optical fiber-optic probes provide fast and accurate temperature measurement, but over much limited temperature range (Luque et al., 2012). The use of metallic thermocouple in microwave cavity is not usually preferred since it can cause electric discharge from metal surface. However, Menéndez et al. (1999) and Liu et al. (2004) suggested that the use of thermocouple in microwave cavity environment can measure accurate temperature provided that it is thin with grounded metal sheath and held exactly at 90° to the electric field component of microwave. In this study, the pyrolysis process temperature was measured by 0.001 m diameter thin probe high temperature K-type thermocouple probes (MgO filled Inconel 600 sheath with grounded junction). The thermocouples were carefully inserted into the upper and middle biomass solid layers to measure the process temperature labeled as TUBL and TMBL, respectively. The thermocouples were connected to Picolog data acquisition system and linked to personal computer for real time monitoring of process temperature. In addition, thermocouples and microwave oven were grounded to avoid inconsistency in temperature reading and arcing during experiment. The microwave irradiation time of 40 min was used. The circulating water temperature was maintained at 2–6oC. The pyrolysis bio-oil yield was determined using Eq. (1).