Pyrolysis of a single biomass pellet involves multiple processes, such as evaporation, decomposition, flow field changes, shrinking or swelling. These processes can be simulated by consolidating pyrolysis theories and computational fluid dynamics. In this paper, a one-dimensional transient model is formulated in a C++ code to simulate the process of pyrolysis of a single biomass pellet. Experimental data from the literature is used to validate this model. This paper presents detailed modelling results for a single biomass pellet under pyrolysis in nitrogen. The model can determine the temperature distribution at different locations at any certain time, and the temperature distribution, at the same location, over time. In this paper, the focus is on how different parameters develop with time within the biomass pellet.