A new method for modeling shrinkage of a biomass particle is presented and validated in a detailed wood pyrolysis model. This model is applied to particle half-thicknesses ranging from 5 pm to 2 cm, temperatures from 800 to 2000 K, and shrinkage factors of 1.0-0.4. Internal temperatures, pyrolysis rates, and yields of tar, light hydrocarbons and char are presented. Based on the results of the model, it is found that shrinkage has a negligible affect on pyrolysis in the thermally thin (Bi < 0.2) and the thermally thick (0.2 less than or equal to Bi less than or equal to 10) pyrolysis regimes. However, in the thermal wave pyrolysis regime (Bi > 10) shrinkage affects both the pyrolysis time and the pyrolysis products. Char shrinkage impacts the pyrolysis process in several ways. These include thinning the pyrolysis reaction region and increasing the pyrolysis temperatures, reducing the residence time of the gases within the particle, and cooling the char layer due to the higher mass flux rates of pyrolysis products. Within the thermal wave pyrolysis regime these effects significantly reduce the light hydrocarbon yield, increase the tar yield, and reduce the pyrolysis time. In the case of a large 2-cm half-thickness poplar particle, with a background temperature of 2000 K and a shrinkage factor of 0.4, inclusion of shrinkage reduces the pyrolysis time by 43%, increases the tar yield from 7% to 13%, decreases the light hydrocarbon yield from 66% to 60%, and decreases the char yield slightly.