AbstractTime‐resolved transient absorption technique is used to investigate the thickness dependence of acoustic phonon modes of silver nanoprisms with two thicknesses, 7.8 ± 1.2 and 8.5 ± 0.69 nm, and a similar bisector length of 31.4 ∼ 31.6 nm. Coherent acoustic phonon signals are observed. A new acoustic phonon frequency within 7.81 cm−1∼ 11.7 cm−1is found and this phonon mode is associated with the thickness of the nanoprism. Another phonon frequency between 1.95 cm−1and 1.71 cm−1is also observed, and its origin can be associated with the bisector length o