Accurate evaluation of the shell elastic modulus of microcapsules is of great significance to understanding their performance during production, processing, and applications. In this work, microcompression was employed to investigate the elastic behaviors of a single microcapsule. It was modeled as a microsphere with a core-shell structure compressed between two rigid plates. Based on the assumption that the contact pressure between the microsphere and plates obeys parabolic distribution, a microcompression method derived from the Reissner's theory and the modified Hertz contact theory was established to evaluate the shell elastic modulus. Applications were carried out on poly(methylmethacrylate) (PMMA) microcapsules containing n-octadecane. The average elastic modulus of PMMA shells measured by the proposed microcompression method agrees well with that of the bulk PMMA sample. Furthermore, the elastic modulus of PMMA shells was found to have size dependence on the diameter of the microcapsules. Finally, finite element models combined with the newly proposed method were constructed to accurately predict the microcompression behaviors of microcapsules with different sizes.