This paper presents evidence that both hydrogens of the ethylenic and phenylic groups of trans-stilbene interact to stabilize the planar structure of the molecule. Also, it proposes a new model that the photoisomerization (PI) arises from the cutting off process of these intramolecular hydrogen interactions (IMHI). The photoisomerization dynamics of stilbene has been investigated by semiempirical quantum chemistry calculations. This new model yields good agreement with experimental results and specifies the PI/IMHI mechanism in detail.