Organic compounds typically do not exhibit room-temperature phosphorescence (RTP) due to the relatively small spin-orbit coupling effect, and the triplet excitons are easily quenched by the efficient molecular motion. Therefore, the regulation of molecular aggregation state, such as host-guest doping, excimer, crystal structure, and polymer, has been employed to suppress the nonradiative transitions and enhance the intersystem crossing efficiency in order to achieve RTP. Among these methods, organic crystals demonstrate an unprecedented advantage as the molecular structure and arrangement within the crystal structure can be accurately resolved by single crystal X-ray diffraction. This allows for the establishment of a structure-property relationship that facilitates exploration of RTP mechanism and the design of efficient RTP materials. The phosphorescent crystals should typically incorporate heavy atoms or functional groups with n-π* transition characteristics to increase the production efficiency of triplet excitons. The regular molecular arrangement within the crystal structure facilitates the stabilization of triplet excitons via delocalization of molecular orbitals. Additionally, the crystalline lattice not only restrains molecular motion to reduce the nonradiative rate but also prevents quenching of triplet excitons inside the crystal structure by oxygen and moisture. This entry briefly discusses the emission mechanism of phosphorescence from organic crystal and summarizes phosphorescent molecular crystals derived from intermolecular hydrogen bond, and incorporation of n-orbitals and heavy atoms, as well as their stimulus-responsive properties, aiming to advance further development of this field.