The properties of biochars produced in biomass pyrolysis processes highly depend on the pyrolysis conditions, specifically the pyrolysis temperature and thermal treatment duration. The higher the temperature and the duration, the more severe is the heat treatment. The present work proposes a new Heat Treatment Severity Index (HTSI) for the quantification of the heat treatment severity during the pyrolysis reaction. This metric takes into account both effects of reactor temperature and heat treatment time inside the reactor. The relevance of this HTSI is assessed through analyzing the evolution of some properties of biochars obtained after pyrolysis of 370 µm beech wood particles in an entrained flow reactor. These biochars were characterized for their chemical composition (elemental analysis), structure (Raman spectroscopy) and reactivity towards oxygen (thermogravimetric analysis). These properties were well correlated with the HTSI following remarkable mathematical relationships. This finding demonstrates the possibility to engineer biochars with controlled properties by a careful mastering of the experimental conditions of the pyrolysis process in terms of temperature and heat treatment time.