Ab initio complete active space self-consistent field (CASSCF) and second-order multireference Moller-Plesset (MRMP2) calculations have been performed to examine the photochemical behavior of diphenylacetylene (DPA) theoretically. The stable structure of DPA in S-0 (S-0-geometry) is optimized to be D-2h. DPA at S-0-geometry is mainly excited into the S-3(B-1u) state and then relaxes into the stable geometry in the B-1u state (B-1u-geometry) which is characterized as a quinoid structure. The B-1u-geometry further relaxes into the globally stable geometry in S-1 (tS(1)-geometry) which takes a trans-bent form. Around tS(1)-geometry, DPA moves into the lowest triplet state through intersystem crossing and finally relaxes into the stable geometry in T-1 with D-2h. The vibrational analyses at the important conformations mentioned above are in good agreement with the experimental findings of time-resolved transient spectroscopy.