The present work examines the impact of the dimensionality of single-particle models (SPMs) for the pyrolysis of thermally thick biomass particles. It builds up comprehensive one- and three-dimensional SPMs and assesses their performances to gain a systematic understanding of the accuracy of predicted results, predictive capability, and computational costs. Cylindrical wheat straw pellets with an average size of 10 mm length and 7 mm diameter were studied on their pyrolysis performances at temperatures ranged from 200 to 700 °C. It investigates the correlation between the inner heating rate of the particle and the resulting irregular porosity distribution in the pyrolyzed particle. The predictions of the models are in good agreement with the experimental measurement, and they show that the predicted char yield is not sensitive to the model dimensionality; however, the rate of mass loss and the char porosity are sensitive to the model dimensionality. Additionally, the three-dimensional SPM is found to be compatible with a wider range of biomass types. The model captures a higher heating rate at the surface and at the center of the particle compared to the region in between. This is linked to an uneven porosity distribution in the pyrolyzed particle, which is higher at the surface and at the center of the particle. While the three-dimensional SPM provides a small improvement in the prediction of the temperature profile compared to the one-dimensional model but offers additional details (e.g., temperature distribution, heating rate, and porosity distribution inside the particle); it requires significantly greater computational times, which might not be justifiable in many situations.