Biomass pyrolysis is a thermochemical process used for renewable products and energies. However, there are still issues that need to be addressed for process modeling and optimization. This study focuses on the relationship between heating rate, shrinkage, and products from flax fibers using thermogravimetric analysis (TGA), microscopic observation, and pyrolysis-gas chromatography/mass spectrometry (Py-GC/MS). TGA confirms sequential evaporation of water then decomposition of hemicellulose, cellulose, and lignin. Observations from the micro-reactor show that flax fibers undergo shrinkage within the temperature range of 335 to 370 °C, depending on the heating rate. Pyrolysis products were analyzed using Py-GC/MS at four different final temperatures from 350 to 500 °C, revealing the presence of anhydrosugars, furans, ketones, phenols, esters, alcohols, aldehydes, and acids. The results indicate a correlation between temperature and the increase in furans and ketones. The analysis suggests that furans and ketones are associated with shrinkage.