This work aims to investigate the damage in ultrahigh‐performance concrete (UHPC) caused by freezing‐thawing action. Freezing‐thawing tests were carried out on UHPCs with and without steel fibers. Mercury intrusion porosimetry (MIP), scanning electron microscopy (SEM), and X‐ray computed tomography (X‐ray CT) were applied to detect the microstructure of the UHPC matrix before and after the freezing‐thawing tests. The results showed that UHPC possessed very excellent freezing‐thawing resistance due to its dense microstructure. After the freezing‐thawing action, cracks occurred and were prone to initiate at the sand‐paste interface in the UHPC matrix. MIP results also indicated that cracks appeared in the UHPC matrix after the freezing‐thawing action. The number of defects that can be seen by X‐ray CT increased in UHPC after the freezing‐thawing action as well. The mismatch of the thermal expansion coefficients of the aggregate and the paste is considered to be the reason for the cracking at the sand‐paste interface. The steel fibers in UHPC inhibited the propagation of cracks in the matrix and improved the freezing‐thawing performance of UHPC.