High pressure can cause rearrangement of atoms in microstructure, and arouses reconfigurations of electronic states, which often brings new materials and new properties. The rich and colorful new phenomena and properties in the typical molecule system have been observed under high pressure. Studies of these new phases and behaviors in typical molecular system under high pressure are very important and significative. Two types of substance, i.e.elementary crystals and small-molecule compounds, are investigated thoroughly on structures and properties under high pressure, and demonstrate various features. In the elementary crystals, such as pressure-induced phase transition and metallization in I₂ (solid iodine) and N₂ (polymeric nitrogen), and in the small-molecule compounds, such as the hydrogen-bond and halogen-bond interactions in crystalline iodoform and bromoform under high pressure, the effects from dihydrogen bonds in ammonia borane which determining the complex dynamics behavior of rotations of the NH₃ and BH₃ groups under high pressure, etc.are discussed deeply. The distinct behaviors of molecular dissociation, reconstruction of crystal structures, and pressure-induced metallization in the typical small-molecule system under high pressure are very different from the ones at ambient conditions. Exploration of the typical small-molecule crystals under high pressure provides an important way to discover the new structures, new materials and new physical phenomena and properties.