Understanding and evaluating crack propagation processes is essential for assessing the performance of concrete under stress. The critical stress intensity factor KIc is one of the key parameters used for describing the fracture resistance of concrete, which plays a crucial role in ensuring the durability of structural members. The RILEM testing method based on the three-point bend test provides standardised guidelines for determining the critical stress intensity factor of concrete and it was used to determine KIc. This study focuses on a comparative analysis of KIc for concretes made using natural gravel aggregate with two different maximum aggregate sizes of 16 mm and 22 mm. An increase of approximately 17% in KIc was observed for concrete with a higher maximum aggregate size (22 mm) compared to concrete with a lower maximum aggregate size (16 mm). In order to explain the differences in the obtained results of KIc, visual analysis of the fracture surfaces and the paths of the main cracks was performed. By comparing the fracture behaviour in the tested concretes, the study aims to contribute to a more comprehensive understanding of the relationship between the aggregate properties and the cracking resistance of concrete.