Alpha-diazocarbonyls play an essential role in electronic microchip production and many other industrial chemistry procedures,which is based upon the photo-induced Wolff rearrangement(WR)to produce ketenes.[1] The WR mechanism,concerted or stepwise,has been extensively studied and argued for more than 100 years.Using state-of-the-art multi-reference states ab initio methods(CASSCF and CASPT2)combined with nonadiabatic dynamics simulation approach(MCTDH),we have investigated the photo-induced WR mechanism of two typical diazo compounds: diazonaphtoquinone(DNQ)and 5-diazo Meldrums acid(DMA),see Figure 1.[2] Our results are in good agreement with the experimental observations[3-4] and clearly show that the WR mechanisms of DNQ and DMA are varied,which are controlled by an intersection seam between the ground state(S0)and the first excited state(S1).Based on the theory of potential energy surface crossings,the deep reasons for the concerted or/and stepwise mechanism and the key factors responsible for various reaction channels will be discussed.