The analysis of concrete microstructure by X-ray CT scans allow to obtain three-dimensional information on the morphology of the pores and fiber distribution. In this work, it was assessed the effect of the addition of polypropylene (PP) fibers in a heated ultra-high performance fiber reinforced concrete (UHPFRC). To this end, three sets of specimens with identical cementitious materials were manufactured at room temperature (RT) and 300^oC: plain concrete (RC), exclusively with steel fibers (DSL), and with steel and PP fibers (DSLPP). It was observed as the addition of PP fibers leads to an increase of the total porosity with respect to the DSL mix, and an increase in the amount of large pores in comparison with RC mix at RT. These effects are beneficial at high temperatures since it was observed as the heating specially affects the small pores and the deleterious effect of melting of the PP fibers is compensated by a lower thermal damage. No significant damage was observed in the matrix of DSLPP mix. Indeed, the mechanical and fracture properties recorded at 300^oC for DSLPP mix were higher than RC and DSL mixes due to lower inside pressure at 300^oC.