Torrefaction is a thermo-chemical process for upgrading biomass that is usually run at temperatures ranging from 200 to more than 300 °C in an oxygen-free atmosphere and at ambient pressure. It is a useful pre-treatment technology for biomass that significantly alters its physical and chemical composition, making it more efficient in pulverized systems. This paper investigates the grindability of eucalyptus chips torrefied at temperatures ranging from 210 °C to 270 °C for a 15 min residence time, compared to raw material before and after fluidization. The tests were carried out in a cold flow fluidization bed chamber with an internal diameter of 150 mm, with air as the fluidizing medium and a chamber height of up to 750 mm. Biomass grindability was assessed by evaluating the change in particle size distribution. The results showed that the degree of thermal degradation depended on the temperature and was also influenced by particle size. The mean particle size of ground torrefied biomass decreased with an increase in torrefaction temperature. Lastly, for larger particles, it was possible to correlate the size distribution with the severity of the heat treatment by a simple linear equation in our experimental conditions.