Experimental investigation of microwave-assisted pyrolysis was successfully done using a specially modified domestic microwave oven. At conditions where no parameterised experimental study was conducted (except temperature measurement), changing other parameters that influence the microwave heating can be investigated qualitatively in detail by means of simulation work. Therefore, a computer based simulation using Comsol Multiphysics software was applied not only to predict how the electromagnetic field distributes within the cavity but also to study different parameters that influence heating distribution inside the microwave oven. The simulation work was initially performed with verification between experimental and simulation temperature profiles at temperature settings of 500 °C and 800 °C and an agreement was achieved in terms of the temperature profile and heating behaviour of the biomass. The simulation work has proved that the inhomogeneity of temperature of the biomass is reflected by the local occurrence of hot spots and cold spots. The effect of different positions of the waveguide is remarkable where the bottom-fed microwave energy oven was shown to have a poor electric field distribution. However, when simulation was done on combining the effect of having the microwave energy fed from the bottom and the presence of the mode stirrer, the electric field was greatly improved with the heating distribution of the biomass resembling that obtained from the side-fed microwaves energy oven (usually refers to a common home microwave oven). The effect of having a mode stirrer rotating inside the microwave oven is also pronounced where the mode stirrer acts to stir the electric field strength within the cavity so that a more uniform heating within the biomass can be achieved. Interestingly from the simulation, for a specified microwave cavity, an optimum bed size of biomass was found at 50 mm height where maximum microwaves energy absorption takes place. In this sense, more microwaves energy can be converted into heat thereby ultimately helping the biomass to reach the desired pyrolysis temperature in shorter time. The COMSOL modelling on microwave heating therefore has shown to be simple and practical for use as a framework in predicting temperature profile of the biomass and intensity of the electric field. •Numerical simulation of biomass heating behavior in a microwave oven.•Temperature inhomogeneity of biomass is due to hot spots and cold spots.•Position of waveguide and the presence of mode stirrer highly influence the electric field distribution.•Simulation results are influenced by the dielectric properties of the biomass.