Mechanically responsive organic crystals that actuate by external stimuli are expected to be applied to actuators and soft robots. Over the past decade, many mechanical crystals have been developed based on photochemical reactions (particularly photoisomerization) and phase transition. However, only a limited number of crystals undergo photoisomerization and phase transition. Furthermore, photoisomerization-induced motion tends to be slow and thick crystals do not actuate. More versatile and faster actuation mechanisms of crystal are required. Recently, we have discovered for the first time very fast crystal actuation (500 Hz) due to the photothermal effect under photoirradiation. Thereafter, for the first time, we have also discovered faster crystal actuation due to the natural vibration induced by the photothermal effect. Furthermore, the natural vibration is greatly amplified by resonance to realize high frequency (700 Hz), large actuation, and high energy conversion efficiency. Both the photothermal effect and natural vibration are universal physical phenomena caused in all crystals that absorb light, and their actuations can be quantitatively simulated. This means that it is now possible to design the operation of crystals. Research on mechanical crystals has reached a new stage where they can be applied as actuation materials to actuators and soft robots.