Numerous studies conducted by researchers have indicated a positive effect of polypropylene fibers (PP fibers) in terms of the reduction of concrete spalling due to fire. Variation of test parameters such as concrete composition, curing conditions, heating curve, aggregate nature and size, etc. creates a challenge for identification of optimal fiber dosage and geometry. The purpose of this work is to study the influence of the fiber length with respect to aggregate size in order to identify optimal fiber length necessary to prevent spalling of concrete. Prisms of cementitious materials of three aggregate sizes (Dmax = 1, 10 and 20 mm) and two fiber lengths (6 and 12 mm) at three fiber dosages (0, 0.5 and 1 kg/m 3) resulting in total of 15 different mixes are prepared. These prisms are unixially loaded at 5 MPa and tested in water saturated condition under ISO 834-1 heating curve. The results show that the maximum aggregate size has an influence on the cracking patterns of the cementitious materials exposed to high temperature. The addition of PP fibers at low dosage produces a positive influence on the reduction of concrete spalling due to fire exposure.