In this work, the influence of diameter of polypropylene (PP) fiber on the spalling of concrete was studied. Concrete prisms of 20 x 20 x 10 cm 3 containing 2 different diameters of fibers (32 and 20 µm) were tested under ISO 834-1 fire curve. In addition to this, tests of residual radial permeability to nitrogen were performed for the same mixes after heating to 80, 150 and 200 °C. Following this, numerical simulations using thermo-mechanical model were performed in order to investigate the influence of the PP fiber diameter on the damage behaviour of cementitious materials. The expansion of fibers during heating can be sufficient for a formation of cracks around the fibers which can increase the permeability and then reduce the risk of concrete instability. The simulations are performed using finite-element code developed in Cast3M. The work carried out made it possible to investigate the influence of PP fiber diameter on the concrete exposed to a high temperature.