As known for the photochemistry of organic molecules,the reactive state is usually not the initially populated one.Ultrafast nonadiabatic transitions (IC or ISC) take place prior to the system reaching the reactive state,through which favorable nonadiabatic reaction pathways are open.We have explored the complicated nonadiabatic reaction pathways for several types of photochemical reactions including the photodissociation and photoisomerization of carbonyl compounds and the DNA photodamage reactions.