The amount of waste biomass and plastic waste that is difficult to decompose, causing accumulation of waste. Therefore, it is necessary to do the right processing to become a renewable energy source. One alternative effort to treat waste that is considered quite perspective is thermal treatment using a microwave oven. This study aims to know the characteristics increasing temperatures during the thermal treatment with final temperature conditions that reach 105 °C. The study using by microwave heating method from a mixture of waste of oil palm fiber biomass and PET plastic waste has been carried out with 800 Watts of power. The mixing composition of the two types of waste varies from 100 %, 75 %, 50 %, 25 % and 0 % by using coconut shell charcoal as an absorber for increasing the level of microwave absorption. Retrieving data on temperature and mass are obtained from recordings per second using data loggers and hyper terminals. The results showed that the value of the heating rate and mass rate has decreased from 1,911 °C/s to 0.490 °C/s and 0.018 g/s to 0.006 g/s. The variations in the composition of the mixture of the two ingredients have effect on decreasing the value of heating rate and mass rate by increasing the composition of PET. The optimum value is obtained in 75% fiber variation with a heating rate of 0.886 ℃/s and a mass rate value of 0.030 g/s.