Flax shive is a highly lignified nonfibrous plant parts obtained from the scutching of straw flax to produce fibers. It is one of the multi-tonnage agricultural wastes. The use of the flax shive as a source for the valuable products is a promising way to solve the problem of the waste processing. This work is devoted to the study of composition and properties of the products of slow and fast pyrolysis of the flax shive. It was found that the slow pyrolysis results in a higher degree of the feedstock decomposition as well as in a higher yield of valuable gases and liquids. The gaseous product yield was found to be over 35 wt. % for the slow pyrolysis, and about 25 wt. % for the fast pyrolysis. For the gaseous products, methane concentration was found to be twice lower for the fast pyrolysis in comparison with the slow one. This decreases the calorific value of gases and leads to the low feasibility of the process. Interestingly, the fast pyrolysis resulted only 20 wt. % of liquids, while for the slow one, a twice increase (ca. 40 wt. %) of liquid yield was obtained. The liquid obtained in a fast mode contained the higher number of carboxylic acids and furanes, while the product obtained in a slow pyrolysis was characterised by the higher concentration of phenols, aldehydes and hydrocarbons increasing its further application. The solid residue obtained by the slow pyrolysis was found to have a higher porosity in comparison with the sample obtained by the fast mode. As the solid residue can be used as an adsorbent, the porosity is a critical parameter to choice the proper mode for flax shive thermal processing.