<p>Shells, a unique component of calcareous sand, exhibit distinct mechanical properties compared to general calcareous sand particles due to their special macrostructures and microstructures. A series of single-particle compression tests are designed to investigate the mechanical properties of 12 types of shells. The failure processes are classified into four modes based on the distribution of minor peaks concerning the major peak in the force–displacement curves. The strength of the shells is calculated using the peak load based on the classical particle strength equation first. The shells with the same macrostructure and microstructure exhibit significant variations in their characteristic strengths and Weibull modulus, suggesting the possible effects of shell thickness. A modified strength eliminating the influence of shell thickness is proposed, and the increase in the Weibull modulus validated the modified strength equation for shells. The relative strength between all the macrostructures and that between all the microstructures are derived based on the modified strength. The particle-scale mechanical properties of shells obtained in this study are useful for understanding the engineering properties of calcareous sands containing shells and deriving relevant constitutive and numerical models.</p>