Diatomites are unicellular algae fossils that can be found worldwide. This paper studies the diatom's shape effect on diatomaceous soil mixtures' dynamic and compressibility properties. Physical characteristics of six diatomite samples with variable shapes and taxonomies are presented. Among these six samples, three diatomites of similar grain size distribution were mixed with kaolin, and the mechanical properties (i.e., shear modulus and damping ratio from small to large strains) were analysed. The results showed that diatom frustules increased the mixtures' compressibility and reduced the maximum shear modulus compared with pure kaolin samples. Moreover, the diatom's shape influenced the normalised shear modulus degradation curve and the damping ratio. Overall, the presence of diatomites reduces the energy dissipation capacity in soils. This research evidenced the importance of distinguishing the shape of diatom in soil mixtures when mechanical properties are analysed, a situation that has yet to be explored.